build: fix warnings in docgen (#711)

Resolved all the outstanding warnings from sphinx.

Going forward, warnings will be treated as errors since the `-W` flag is being used with `sphinx-build` 
diff --git a/docs/Makefile b/docs/Makefile
deleted file mode 100644
index 4887d62..0000000
--- a/docs/Makefile
+++ /dev/null
@@ -1,225 +0,0 @@
-# Makefile for Sphinx documentation
-#
-
-# You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
-BUILDDIR      = _build
-
-# Internal variables.
-PAPEROPT_a4     = -D latex_paper_size=a4
-PAPEROPT_letter = -D latex_paper_size=letter
-ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
-# the i18n builder cannot share the environment and doctrees with the others
-I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
-
-.PHONY: help
-help:
-	@echo "Please use \`make <target>' where <target> is one of"
-	@echo "  html       to make standalone HTML files"
-	@echo "  dirhtml    to make HTML files named index.html in directories"
-	@echo "  singlehtml to make a single large HTML file"
-	@echo "  pickle     to make pickle files"
-	@echo "  json       to make JSON files"
-	@echo "  htmlhelp   to make HTML files and a HTML help project"
-	@echo "  qthelp     to make HTML files and a qthelp project"
-	@echo "  applehelp  to make an Apple Help Book"
-	@echo "  devhelp    to make HTML files and a Devhelp project"
-	@echo "  epub       to make an epub"
-	@echo "  epub3      to make an epub3"
-	@echo "  latex      to make LaTeX files, you can set PAPER=a4 or PAPER=letter"
-	@echo "  latexpdf   to make LaTeX files and run them through pdflatex"
-	@echo "  latexpdfja to make LaTeX files and run them through platex/dvipdfmx"
-	@echo "  text       to make text files"
-	@echo "  man        to make manual pages"
-	@echo "  texinfo    to make Texinfo files"
-	@echo "  info       to make Texinfo files and run them through makeinfo"
-	@echo "  gettext    to make PO message catalogs"
-	@echo "  changes    to make an overview of all changed/added/deprecated items"
-	@echo "  xml        to make Docutils-native XML files"
-	@echo "  pseudoxml  to make pseudoxml-XML files for display purposes"
-	@echo "  linkcheck  to check all external links for integrity"
-	@echo "  doctest    to run all doctests embedded in the documentation (if enabled)"
-	@echo "  coverage   to run coverage check of the documentation (if enabled)"
-	@echo "  dummy      to check syntax errors of document sources"
-
-.PHONY: clean
-clean:
-	rm -rf $(BUILDDIR)/*
-
-.PHONY: html
-html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
-	@echo
-	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
-
-.PHONY: dirhtml
-dirhtml:
-	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml
-	@echo
-	@echo "Build finished. The HTML pages are in $(BUILDDIR)/dirhtml."
-
-.PHONY: singlehtml
-singlehtml:
-	$(SPHINXBUILD) -b singlehtml $(ALLSPHINXOPTS) $(BUILDDIR)/singlehtml
-	@echo
-	@echo "Build finished. The HTML page is in $(BUILDDIR)/singlehtml."
-
-.PHONY: pickle
-pickle:
-	$(SPHINXBUILD) -b pickle $(ALLSPHINXOPTS) $(BUILDDIR)/pickle
-	@echo
-	@echo "Build finished; now you can process the pickle files."
-
-.PHONY: json
-json:
-	$(SPHINXBUILD) -b json $(ALLSPHINXOPTS) $(BUILDDIR)/json
-	@echo
-	@echo "Build finished; now you can process the JSON files."
-
-.PHONY: htmlhelp
-htmlhelp:
-	$(SPHINXBUILD) -b htmlhelp $(ALLSPHINXOPTS) $(BUILDDIR)/htmlhelp
-	@echo
-	@echo "Build finished; now you can run HTML Help Workshop with the" \
-	      ".hhp project file in $(BUILDDIR)/htmlhelp."
-
-.PHONY: qthelp
-qthelp:
-	$(SPHINXBUILD) -b qthelp $(ALLSPHINXOPTS) $(BUILDDIR)/qthelp
-	@echo
-	@echo "Build finished; now you can run "qcollectiongenerator" with the" \
-	      ".qhcp project file in $(BUILDDIR)/qthelp, like this:"
-	@echo "# qcollectiongenerator $(BUILDDIR)/qthelp/google-auth.qhcp"
-	@echo "To view the help file:"
-	@echo "# assistant -collectionFile $(BUILDDIR)/qthelp/google-auth.qhc"
-
-.PHONY: applehelp
-applehelp:
-	$(SPHINXBUILD) -b applehelp $(ALLSPHINXOPTS) $(BUILDDIR)/applehelp
-	@echo
-	@echo "Build finished. The help book is in $(BUILDDIR)/applehelp."
-	@echo "N.B. You won't be able to view it unless you put it in" \
-	      "~/Library/Documentation/Help or install it in your application" \
-	      "bundle."
-
-.PHONY: devhelp
-devhelp:
-	$(SPHINXBUILD) -b devhelp $(ALLSPHINXOPTS) $(BUILDDIR)/devhelp
-	@echo
-	@echo "Build finished."
-	@echo "To view the help file:"
-	@echo "# mkdir -p $$HOME/.local/share/devhelp/google-auth"
-	@echo "# ln -s $(BUILDDIR)/devhelp $$HOME/.local/share/devhelp/google-auth"
-	@echo "# devhelp"
-
-.PHONY: epub
-epub:
-	$(SPHINXBUILD) -b epub $(ALLSPHINXOPTS) $(BUILDDIR)/epub
-	@echo
-	@echo "Build finished. The epub file is in $(BUILDDIR)/epub."
-
-.PHONY: epub3
-epub3:
-	$(SPHINXBUILD) -b epub3 $(ALLSPHINXOPTS) $(BUILDDIR)/epub3
-	@echo
-	@echo "Build finished. The epub3 file is in $(BUILDDIR)/epub3."
-
-.PHONY: latex
-latex:
-	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
-	@echo
-	@echo "Build finished; the LaTeX files are in $(BUILDDIR)/latex."
-	@echo "Run \`make' in that directory to run these through (pdf)latex" \
-	      "(use \`make latexpdf' here to do that automatically)."
-
-.PHONY: latexpdf
-latexpdf:
-	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
-	@echo "Running LaTeX files through pdflatex..."
-	$(MAKE) -C $(BUILDDIR)/latex all-pdf
-	@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."
-
-.PHONY: latexpdfja
-latexpdfja:
-	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
-	@echo "Running LaTeX files through platex and dvipdfmx..."
-	$(MAKE) -C $(BUILDDIR)/latex all-pdf-ja
-	@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."
-
-.PHONY: text
-text:
-	$(SPHINXBUILD) -b text $(ALLSPHINXOPTS) $(BUILDDIR)/text
-	@echo
-	@echo "Build finished. The text files are in $(BUILDDIR)/text."
-
-.PHONY: man
-man:
-	$(SPHINXBUILD) -b man $(ALLSPHINXOPTS) $(BUILDDIR)/man
-	@echo
-	@echo "Build finished. The manual pages are in $(BUILDDIR)/man."
-
-.PHONY: texinfo
-texinfo:
-	$(SPHINXBUILD) -b texinfo $(ALLSPHINXOPTS) $(BUILDDIR)/texinfo
-	@echo
-	@echo "Build finished. The Texinfo files are in $(BUILDDIR)/texinfo."
-	@echo "Run \`make' in that directory to run these through makeinfo" \
-	      "(use \`make info' here to do that automatically)."
-
-.PHONY: info
-info:
-	$(SPHINXBUILD) -b texinfo $(ALLSPHINXOPTS) $(BUILDDIR)/texinfo
-	@echo "Running Texinfo files through makeinfo..."
-	make -C $(BUILDDIR)/texinfo info
-	@echo "makeinfo finished; the Info files are in $(BUILDDIR)/texinfo."
-
-.PHONY: gettext
-gettext:
-	$(SPHINXBUILD) -b gettext $(I18NSPHINXOPTS) $(BUILDDIR)/locale
-	@echo
-	@echo "Build finished. The message catalogs are in $(BUILDDIR)/locale."
-
-.PHONY: changes
-changes:
-	$(SPHINXBUILD) -b changes $(ALLSPHINXOPTS) $(BUILDDIR)/changes
-	@echo
-	@echo "The overview file is in $(BUILDDIR)/changes."
-
-.PHONY: linkcheck
-linkcheck:
-	$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
-	@echo
-	@echo "Link check complete; look for any errors in the above output " \
-	      "or in $(BUILDDIR)/linkcheck/output.txt."
-
-.PHONY: doctest
-doctest:
-	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
-	@echo "Testing of doctests in the sources finished, look at the " \
-	      "results in $(BUILDDIR)/doctest/output.txt."
-
-.PHONY: coverage
-coverage:
-	$(SPHINXBUILD) -b coverage $(ALLSPHINXOPTS) $(BUILDDIR)/coverage
-	@echo "Testing of coverage in the sources finished, look at the " \
-	      "results in $(BUILDDIR)/coverage/python.txt."
-
-.PHONY: xml
-xml:
-	$(SPHINXBUILD) -b xml $(ALLSPHINXOPTS) $(BUILDDIR)/xml
-	@echo
-	@echo "Build finished. The XML files are in $(BUILDDIR)/xml."
-
-.PHONY: pseudoxml
-pseudoxml:
-	$(SPHINXBUILD) -b pseudoxml $(ALLSPHINXOPTS) $(BUILDDIR)/pseudoxml
-	@echo
-	@echo "Build finished. The pseudo-XML files are in $(BUILDDIR)/pseudoxml."
-
-.PHONY: dummy
-dummy:
-	$(SPHINXBUILD) -b dummy $(ALLSPHINXOPTS) $(BUILDDIR)/dummy
-	@echo
-	@echo "Build finished. Dummy builder generates no files."
diff --git a/docs/conf.py b/docs/conf.py
index db1872e..b68467f 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -369,7 +369,7 @@
     "python": ("https://docs.python.org/3.5", None),
     "urllib3": ("https://urllib3.readthedocs.io/en/stable", None),
     "requests": ("https://requests.kennethreitz.org/en/master/", None),
-    "requests-oauthlib": ("http://requests-oauthlib.readthedocs.io/en/stable", None),
+    "requests-oauthlib": ("https://requests-oauthlib.readthedocs.io/en/stable/", None),
 }
 
 # Autodoc config
diff --git a/docs/reference/google.auth.credentials_async.rst b/docs/reference/google.auth._credentials_async.rst
similarity index 100%
rename from docs/reference/google.auth.credentials_async.rst
rename to docs/reference/google.auth._credentials_async.rst
diff --git a/docs/reference/google.auth.jwt_async.rst b/docs/reference/google.auth._jwt_async.rst
similarity index 75%
rename from docs/reference/google.auth.jwt_async.rst
rename to docs/reference/google.auth._jwt_async.rst
index 4e56a6e..d27984b 100644
--- a/docs/reference/google.auth.jwt_async.rst
+++ b/docs/reference/google.auth._jwt_async.rst
@@ -1,7 +1,7 @@
 google.auth.jwt\_async module
 =============================
 
-.. automodule:: google.auth.jwt_async
+.. automodule:: google.auth._jwt_async
    :members:
    :inherited-members:
    :show-inheritance:
diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index e21eaf9..eb8328a 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -33,4 +33,4 @@
    google.auth.identity_pool
    google.auth.impersonated_credentials
    google.auth.jwt
-   google.auth.jwt_async
+   google.auth._jwt_async
diff --git a/docs/reference/google.auth.transport.aiohttp_requests.rst b/docs/reference/google.auth.transport._aiohttp_requests.rst
similarity index 100%
rename from docs/reference/google.auth.transport.aiohttp_requests.rst
rename to docs/reference/google.auth.transport._aiohttp_requests.rst
diff --git a/docs/reference/google.oauth2.credentials_async.rst b/docs/reference/google.oauth2._credentials_async.rst
similarity index 100%
rename from docs/reference/google.oauth2.credentials_async.rst
rename to docs/reference/google.oauth2._credentials_async.rst
diff --git a/docs/reference/google.oauth2.service_account_async.rst b/docs/reference/google.oauth2._service_account_async.rst
similarity index 100%
rename from docs/reference/google.oauth2.service_account_async.rst
rename to docs/reference/google.oauth2._service_account_async.rst
diff --git a/docs/user-guide.rst b/docs/user-guide.rst
index b315cc9..6674c5a 100644
--- a/docs/user-guide.rst
+++ b/docs/user-guide.rst
@@ -520,7 +520,6 @@
 .. _App Engine: https://cloud.google.com/appengine/
 .. _Cloud Functions: https://cloud.google.com/functions/
 .. _Cloud Run: https://cloud.google.com/run/
-.. _Compute Engine: https://cloud.google.com/compute/
 .. _Identity Aware Proxy: https://cloud.google.com/iap/
 .. _Google OpenID Connect: https://developers.google.com/identity/protocols/OpenIDConnect
 .. _Google ID Token: https://developers.google.com/identity/protocols/OpenIDConnect#validatinganidtoken