Clarify create_engine encoding; update cx_Oracle

Clarify create_engine encoding; update cx_Oracle

The create_engine()->encoding parameter is mostly irrelevant
under Python 3.  make it clear this parameter is not generally
useful anymore and refer readers to the dialect documenation.

Update cx_Oracle documentation to feature many examples of
the encoding / nencoding parameters, remove extra detail that
is not generally useful and reorganize information into
more specific sections, de-emphasizing legacy / Python 2
specific sections.

Change-Id: I42dafb85de0a585c9e05e1e3d787d8d6bfa632c0