allow legacy forms with __allow_unmapped__

allow legacy forms with __allow_unmapped__

Improved support for legacy 1.4 mappings that use annotations which don't
include ``Mapped[]``, by ensuring the ``__allow_unmapped__`` attribute can
be used to allow such legacy annotations to pass through Annotated
Declarative without raising an error and without being interpreted in an
ORM runtime context. Additionally improved the error message generated when
this condition is detected, and added more documentation for how this
situation should be handled. Unfortunately the 1.4 WARN_SQLALCHEMY_20
migration warning cannot detect this particular configurational issue at
runtime with its current architecture.

Fixes: #8692
Change-Id: I5c642bcc1ebb7816f9470ec9bb0951550b6d55f1