more many-to-one typing

more many-to-one typing

since we are typing centric, note this configuration
as we have just supported in #8668.

Note also I am just taking "backref" out of the basic
version of the docs here totally, this doc is already
a lot to read / take in without making it even more
confusing; backref still has an entirely dedicated
docs page which can have all the additional behaviors
of backref() described.

Additionally, get other "optional" forms to work including
``cls | None`` and ``Union[cls, None]``.

Fixes: #8668
Change-Id: I2b026f496a1710ddebfb4aa6cf8459b4892cbc54