Improve descriptions of introspection changes

Improve descriptions of introspection changes

Several of the introspection changes in Python 3.4 are indirect,
where inspect module changes affected pydoc, and those in turn
affected the help builtin. This update adds versionchanged
notes in the key locations, as well as more coverage in the
What's New document (in particular, a note in the porting
section regarding the expanded domain for inspect.getfullargspec).