objects, such that they were prevented from being called outside
of the declarative process; this is related to the enhancements
of #3150 which allow ``@declared_attr`` to return a value that is
cached based on the current class as it's being configured.
The exception raise has been removed, and the behavior changed
so that outside of the declarative process, the function decorated by
``@declared_attr`` is called every time just like a regular
``@property``, without using any caching, as none is available
at this stage.
fixes #3331

of a declarative inheritance hierarchy would prevent attributes
and configuration being correctly propagated from the base class
to the inheriting class.
fixes #3219 fixes #3240

subclass, the value returned by the descriptor is not available
because the superclass is already mapped with the InstrumentedAttribute,
until the subclass is mapped.  We add a setattr() to set up that
attribute so that the __mapper_args__ hook and possibly others
have access to the "cascaded" version of the attribute within
the call.

and extensions into an external library, and also reorganizes most large documentation
pages into many small areas to reduce scrolling and better present the context
into a more fine-grained hierarchy.

when using :class:`.AbstractConcreteBase` in conjunction with
a subclass that declares ``__abstract__``.
fixes #3185