Replaced variable o with obj in operator.rst because o is easy to
confuse.
Added a note about Python 3's collections.Mapping etc., above section
that describes isMappingType() etc.
Added xrefs between os, os.path, fileinput, and open().
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 2f4685d..39ada43 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -765,7 +765,8 @@
Python enforces that the mode, after stripping ``'U'``, begins with ``'r'``,
``'w'`` or ``'a'``.
- See also the :mod:`fileinput` module.
+ See also the :mod:`fileinput` module, the :mod:`os` module, and the
+ :mod:`os.path` module.
.. versionchanged:: 2.5
Restriction on first letter of mode string introduced.