#18600: add policy to add_string, and as_bytes and __bytes__ methods.

This was triggered by wanting to make the doctest in email.policy.rst pass;
as_bytes and __bytes__ are clearly useful now that we have BytesGenerator.
Also updated the Message docs to document the policy keyword that was
added in 3.3.
#18600: In 3.3, as_string does not accept a policy keyword.

Also, document the policy keyword that was added to Message in 3.3.
Null merge related to #18600.

For 3.3, we just deleted the example.  In 3.4 it was fixed.