Merge branch '12488-add-remove-parent-epic-quick-actions-doc-ce' into 'master'

Add documentation on quick actions for add/remove epic parent relations.

See merge request gitlab-org/gitlab-ce!30174

doc/user/project/quick_actions.md

@@ -88,3 +88,5 @@ The following quick actions are applicable for epics threads and description:
 | `/relabel ~label1 ~label2` | Replace label                           |
 | <code>/child_epic &lt;&epic &#124;  group&epic &#124; Epic URL&gt;</code> | Adds child epic to epic ([introduced in GitLab 12.0](https://gitlab.com/gitlab-org/gitlab-ee/issues/7330)) |
 | <code>/remove_child_epic &lt;&epic &#124; group&epic &#124; Epic URL&gt;</code> | Removes child epic from epic ([introduced in GitLab 12.0](https://gitlab.com/gitlab-org/gitlab-ee/issues/7330)) |
+| <code>/parent_epic &lt;&epic &#124;  group&epic &#124; Epic URL&gt;</code> | Sets parent epic to epic ([introduced in GitLab 12.1](https://gitlab.com/gitlab-org/gitlab-ee/issues/10556)) |
+| <code>/remove_parent_epic | Removes parent epic from epic ([introduced in GitLab 12.1](https://gitlab.com/gitlab-org/gitlab-ee/issues/10556)) |