[3.10] bpo-28516: document contextlib.ExitStack.__enter__ behavior (GH-31636) (GH-32171)

The enter_context is updated with following information: 'The :meth:`__enter__` method
      returns the ExitStack instance, and performs no additional operations.'

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 86384cf83f96fcaec03e2ad6516e2e24f20d3b92)

Co-authored-by: vidhya <96202776+Vidhyavinu@users.noreply.github.com>