Keystone and others depend on the .property attribute being
"mirrored" when a @hybrid_property is linked directly to a
mapped attribute.  Restore this linkage and also create a defined
behavior for the .info dictionary; it is that of the hybrid itself.
Add this behavioral change to the migration notes.

Change-Id: I8ac34ef52039387230c648866c5ca15d381f7fee
References: #3653

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

to the new MutableList and MutableSet classes, fixes #3297

from https://bitbucket.org/zzzeek/sqlalchemy/issues/3297

that this structure is only intended to track additions
and removals from the dictionary, not recursive tracking
of embedded changes.  fixes #3646.

- remove redundant concrete helper docs from declarative docs,
two places is enough

when cross-compiling from DDL to SQL, fixes #3615