swig-py: Belatedly document importing fully-qualified C function names.

* subversion/bindings/swig/INSTALL
    Teak wording.

* subversion/bindings/swig/python/README
    Document the alternatives in detail.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1457815 13f79535-47bb-0310-9956-ffa450edef68