Honor hybrid property / method docstrings

Honor hybrid property / method docstrings

The docstring specified on a hybrid property or method is now honored
at the class level, allowing it to work with tools like Sphinx
autodoc.  The mechanics here necessarily involve some wrapping of
expressions to occur for hybrid properties, which may cause them
to appear differently using introspection.

Fixes: #3653
Change-Id: I02549977fe8b2a051802eed7b00cc532fbc214e3
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/239