MR pipelines cannot be skipped

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

doc/user/project/push_options.md

@@ -35,7 +35,7 @@ You can use push options to skip a CI/CD pipeline, or pass CI/CD variables.
 
 | Push option                    | Description                                                                                 | Introduced in version |
 | ------------------------------ | ------------------------------------------------------------------------------------------- |---------------------- |
-| `ci.skip`                      | Do not create a CI pipeline for the latest push.                                            | [11.7](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/15643) |
+| `ci.skip`                      | Do not create a CI pipeline for the latest push. Only skips branch pipelines and not [merge request pipelines](../../ci/pipelines/merge_request_pipelines.md). | [11.7](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/15643) |
 | `ci.variable="<name>=<value>"` | Provide [CI/CD variables](../../ci/variables/index.md) to be used in a CI pipeline, if one is created due to the push. | [12.6](https://gitlab.com/gitlab-org/gitlab/-/issues/27983) |
 
 An example of using `ci.skip`: