[docs] [issue20476] If new email policies are used, default message factory should be EmailMessage

[Roundup Robot]

Roundup Robot added the comment:

New changeset 9ba8f4be0651 by R David Murray in branch 'default':
#20476: add a message_factory policy attribute to email.
https://hg.python.org/cpython/rev/9ba8f4be0651

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