gh-92082: contextlib docs: Change aclosing from a class to a function for consistency (GH-92155)

Signed-off-by: prwatson <prwatson@redhat.com>
(cherry picked from commit 958f21c5cdb3bbbd16fec87164785cff3dacce96)

Co-authored-by: Thaddeus1499 <104600742+Thaddeus1499@users.noreply.github.com>