t, doc: update tests, reference for "--force-if-includes"

t, doc: update tests, reference for "--force-if-includes"

Update test cases for the new option, and document its usage
and update related references.

Update test cases for the new option, and document its usage
and update related references.

 - t/t5533-push-cas.sh:
   Update test cases for "compare-and-swap" when used along with
   "--force-if-includes" helps mitigate overwrites when remote
   refs are updated in the background; allows forced updates when
   changes from remote are integrated locally.

 - Documentation:
   Add reference for the new option, configuration setting
   ("push.useForceIfIncludes") and advise messages.

Signed-off-by: Srinidhi Kaushik <shrinidhi.kaushik@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>