Issue #14783: Improve int() docstring and also str(), range(), and slice().

This commit rewrites the docstring for int() to incorporate the documentation
changes made in issue #16036.  It also switches the docstrings for int(),
str(), range(), and slice() to use multi-line signatures.
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 1ee8540..2575d7f 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1236,7 +1236,8 @@
    standard type hierarchy in :ref:`types`.
 
 
-.. function:: str([object[, encoding[, errors]]])
+.. function:: str(object='')
+              str(object[, encoding[, errors]])
 
    Return a string version of an object, using one of the following modes: