Merge branch 'api-subscribe-doc' into 'master'

Use POST in the endpoint description for :resource/unsubscribe

See merge request !9469

doc/api/issues.md

@@ -569,7 +569,7 @@ from it. If the user is not subscribed to the issue, the
 status code `304` is returned.
 
 ```
-DELETE /projects/:id/issues/:issue_id/unsubscribe
+POST /projects/:id/issues/:issue_id/unsubscribe
 ```
 
 | Attribute | Type | Required | Description |

doc/api/labels.md

@@ -228,7 +228,7 @@ from it. If the user is not subscribed to the label, the
 status code `304` is returned.
 
 ```
-DELETE /projects/:id/labels/:label_id/unsubscribe
+POST /projects/:id/labels/:label_id/unsubscribe
 ```
 
 | Attribute  | Type              | Required | Description                          |

doc/api/merge_requests.md

@@ -741,7 +741,7 @@ notifications from that merge request. If the user is
 not subscribed to the merge request, the status code `304` is returned.
 
 ```
-DELETE /projects/:id/merge_requests/:merge_request_id/unsubscribe
+POST /projects/:id/merge_requests/:merge_request_id/unsubscribe
 ```
 
 | Attribute | Type | Required | Description |