Document the Delete Merged Branches functionality

* Note that protected branches will not be part of the deletion

changelogs/unreleased/33445-document-delete-merge-branches-won-t-touch-protected-branches-docs.yml

+---
+title: Document the Delete Merged Branches functionality
+merge_request:
+author:

doc/api/branches.md

@@ -251,6 +251,8 @@ curl --request DELETE --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" "https://gi
 
 Will delete all branches that are merged into the project's default branch.
 
+Protected branches will not be deleted as part of this operation.
+
 ```
 DELETE /projects/:id/repository/merged_branches
 ```

doc/user/project/branches.md

+# Branches
+
+## Delete Merged Branches
+
+> [Introduced][ce-6449] in GitLab 8.14.
+
+![Delete Merged Branches](img/delete_merged_branches.png)
+
+This feature allows merged branches to be deleted in bulk. Only branches that have been merged and are not protected will be deleted as part of this operation.
+
+It's particularly useful to clean up old branches that were not deleting automatically when a Merge Request was merged. 
+
+---
+
+[ce-6449]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6449 "Add button to delete all merged branches"