Help: Update maintainer edit checkbox label

Recent GitLab updates have changed the label associated with the
checkbox in merge request descriptions for allowing edits from
maintainers. Make the contributor guidelines match the new label
and add a note for why it is recommended.

CONTRIBUTING.rst

@@ -36,7 +36,9 @@ To contribute patches:
 #. Push a topic branch to a personal repository fork on GitLab.
 #. Create a GitLab Merge Request targeting the upstream ``master`` branch
    (even if the change is intended for merge to the ``release`` branch).
-   Check the box to allow edits from maintainers.
+   Check the box labelled "Allow commits from members who can merge to the
+   target branch".  This will allow maintainers to make minor edits on your
+   behalf.
 
 The merge request will enter the `CMake Review Process`_ for consideration.