[3.12] gh-118455: Fix mangle_from_ default value in email.policy.Policy.__doc__ (GH-118456) (#118586)

gh-118455: Fix mangle_from_ default value in email.policy.Policy.__doc__ (GH-118456)

* Fix mangle_from_ default value in email.policy.Policy.__doc__

The docstring says it defaults to True, but it actually defaults
to False. Only the Compat32 subclass overrides that.

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(cherry picked from commit fed8d73fde779fca41026398376cb3038e9b2b5f)

Co-authored-by: wim glenn <jump@wimglenn.com>
Co-authored-by: Nikita Sobolev <mail@sobolevn.me>