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

#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.