Add "Manage merge requests" permission to docs

doc/permissions/permissions.md

@@ -17,6 +17,7 @@ If a user is a GitLab administrator they receive all permissions.
 | Create code snippets                  |         | ✓          | ✓           | ✓        | ✓      |
 | Manage issue tracker                  |         | ✓          | ✓           | ✓        | ✓      |
 | Manage labels                         |         | ✓          | ✓           | ✓        | ✓      |
+| Manage merge requests                 |         |            | ✓           | ✓        | ✓      |
 | Create new merge request              |         |            | ✓           | ✓        | ✓      |
 | Create new branches                   |         |            | ✓           | ✓        | ✓      |
 | Push to non-protected branches        |         |            | ✓           | ✓        | ✓      |