Servlet to purge old rooms (#5845)

docs/admin_api/purge_room.md

@@ -0,0 +1,18 @@
+Purge room API
+==============
+
+This API will remove all trace of a room from your database.
+
+All local users must have left the room before it can be removed.
+
+The API is:
+
+```
+POST /_synapse/admin/v1/purge_room
+
+{
+    "room_id": "!room:id"
+}
+```
+
+You must authenticate using the access token of an admin user.