Warnings for @declared_attr.cascading

Warnings for @declared_attr.cascading

A warning is emitted if a subclass attempts to override an attribute
that was declared on a superclass using ``@declared_attr.cascading``
that the overridden attribute will be ignored. This use
case cannot be fully supported down to further subclasses without more
complex development efforts, so for consistency the "cascading" is
honored all the way down regardless of overriding attributes.

A warning is emitted if the ``@declared_attr.cascading`` attribute is
used with a special declarative name such as ``__tablename__``, as this
has no effect.

Ensure that documenation refers to the current inconsistency that
__tablename__ can be overridden by subclasses however
@declared_attr.cascading cannot.

Fixes: #4091
Fixes: #4092
Change-Id: I3aecdb2f99d408e404a1223f5ad86ae3c7fdf036