bpo-42958: Improve description of shallow= in filecmp.cmp docs (GH-27166)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Alexander Vandenbulcke <alexander.vandenbulcke95@gmail.com>
(cherry picked from commit a8dc4893d2b28827e82447326ea47759c161a722)

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>