Increase default join burst ratelimiting (#9674)

It's legitimate behaviour to try and join a bunch of rooms at once.

changelog.d/9674.misc

+Increase default join ratelimiting burst rate.

docs/sample_config.yaml

@@ -869,10 +869,10 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #rc_joins:
 #  local:
 #    per_second: 0.1
-#    burst_count: 3
+#    burst_count: 10
 #  remote:
 #    per_second: 0.01
-#    burst_count: 3
+#    burst_count: 10
 #
 #rc_3pid_validation:
 #  per_second: 0.003

synapse/config/ratelimiting.py

@@ -95,11 +95,11 @@ class RatelimitConfig(Config):
 
         self.rc_joins_local = RateLimitConfig(
             config.get("rc_joins", {}).get("local", {}),
-            defaults={"per_second": 0.1, "burst_count": 3},
+            defaults={"per_second": 0.1, "burst_count": 10},
         )
         self.rc_joins_remote = RateLimitConfig(
             config.get("rc_joins", {}).get("remote", {}),
-            defaults={"per_second": 0.01, "burst_count": 3},
+            defaults={"per_second": 0.01, "burst_count": 10},
         )
 
         # Ratelimit cross-user key requests:
@@ -187,10 +187,10 @@ class RatelimitConfig(Config):
         #rc_joins:
         #  local:
         #    per_second: 0.1
-        #    burst_count: 3
+        #    burst_count: 10
         #  remote:
         #    per_second: 0.01
-        #    burst_count: 3
+        #    burst_count: 10
         #
         #rc_3pid_validation:
         #  per_second: 0.003