Fixed case in form widgets docs
Fixed #21649 -- Added optional invalidation of sessions when user password changes.

Thanks Paul McMillan, Aymeric Augustin, and Erik Romijn for reviews.
Fixed #22236 -- Removed inappropriate usage of signals

Thanks Aymeric Augustin for the report and Tim Graham for the review.
Fixed #22216 -- Added Form.non_field_errors to form API doc.
Fixed #21977 -- Deprecated SimpleTestCase.urls