write some docs for the quarantine_media api (#6458)

docs/admin_api/media_admin_api.md

@@ -21,3 +21,20 @@ It returns a JSON body like the following:
     ]
 }
 ```
+
+# Quarantine media in a room
+
+This API 'quarantines' all the media in a room.
+
+The API is:
+
+```
+POST /_synapse/admin/v1/quarantine_media/<room_id>
+
+{}
+```
+
+Quarantining media means that it is marked as inaccessible by users. It applies
+to any local media, and any locally-cached copies of remote media.
+
+The media file itself (and any thumbnails) is not deleted from the server.