Refine ambiguous access for unknown attribute types

Refine ambiguous access for unknown attribute types

Restored instance-level support for plain Python descriptors, e.g.
``@property`` objects, in conjunction with association proxies, in that if
the proxied object is not within ORM scope at all, it gets classified as
"ambiguous" but is proxed directly.  For class level access, a basic class
level``__get__()`` now returns the
:class:`.AmbiguousAssociationProxyInstance` directly, rather than raising
its exception, which is the closest approximation to the previous behavior
that returned the :class:`.AssociationProxy` itself that's possible.  Also
improved the stringification of these objects to be more descriptive of
current state.

Fixes: #4574
Change-Id: I787a22806b5530c146ae6ee66b588e5b191ae689