Add transactional API to history purge

Make the purge request return quickly, and allow scripts to poll for updates.

docs/admin_api/purge_history_api.rst

@@ -32,3 +32,30 @@ specified by including an event_id in the URI, or by setting a
 id is given, that event (and others at the same graph depth) will be retained.
 If ``purge_up_to_ts`` is given, it should be a timestamp since the unix epoch,
 in milliseconds.
+
+The API starts the purge running, and returns immediately with a JSON body with
+a purge id:
+
+.. code:: json
+
+    {
+        "purge_id": "<opaque id>"
+    }
+
+Purge status query
+------------------
+
+It is possible to poll for updates on recent purges with a second API;
+
+``GET /_matrix/client/r0/admin/purge_history_status/<purge_id>``
+
+(again, with a suitable ``access_token``). This API returns a JSON body like
+the following:
+
+.. code:: json
+
+    {
+        "status": "active"
+    }
+
+The status will be one of ``active``, ``complete``, or ``failed``.