Documentation: fix push options capabilities

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31068 forgot to
add a short description of the new push options that it added.

doc/user/project/merge_requests/index.md

@@ -287,6 +287,8 @@ as pushing changes:
 - Set the target of the merge request to a particular branch.
 - Set the merge request to merge when its pipeline succeeds.
 - Set the merge request to remove the source branch when it's merged.
+- Set the title of the merge request to a particular title.
+- Set the description of the merge request to a particular description.
 
 ### Create a new merge request using git push options