Due to the implementation nature the ci.skip push option
is only processed for branch pipelines
(which carry a push context) and are not considered
in merge request pipelines
(which carry a full branch context).

This change adds an explicit behavior note to the
push options documentation for `ci.skip` to make
this unsurprising to users of merge request
pipelines.

Related issue:
https://gitlab.com/gitlab-org/gitlab/-/issues/27955