Rate limit joins per-room (#13276)

docs/usage/configuration/config_documentation.md

@@ -1471,6 +1471,25 @@ rc_joins:
     per_second: 0.03
     burst_count: 12
 ```
+---
+### `rc_joins_per_room`
+
+This option allows admins to ratelimit joins to a room based on the number of recent
+joins (local or remote) to that room. It is intended to mitigate mass-join spam
+waves which target multiple homeservers.
+
+By default, one join is permitted to a room every second, with an accumulating
+buffer of up to ten instantaneous joins.
+
+Example configuration (default values):
+```yaml
+rc_joins_per_room:
+  per_second: 1
+  burst_count: 10
+```
+
+_Added in Synapse 1.64.0._
+
 ---
 ### `rc_3pid_validation`