Return the assigned value in MultableDict.setdefault

distinguished for when it was actually the value ``False``.
The ``__abstract__`` flag needs to acutally evaluate to a True
value at the level being tested.
fixes #3097

mapper is implicitly combining one of its column-based attributes
with that of the parent, where those columns normally don't necessarily
share the same value.  This is an extension of an existing check that
was added via :ticket:`1892`; however this new check emits only a
warning, instead of an exception, to allow for applications that may
be relying upon the existing behavior.
fixes #3042

mixin or abstract class when accessed, so that modifications made
to this dictionary by declarative itself won't conflict with that
of other mappings.  The dictionary is modified regarding the
``version_id_col`` and ``polymorphic_on`` arguments, replacing the
column within with the one that is officially mapped to the local
class/table. fixes #3062

report change events for the ``setdefault()`` dictionary operation.
fixes #3051

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.

No clue.  nosetests doesn't do this.  concerning that other tests might have the same issue.

Found using: https://github.com/intgr/topy