Give an estimate for room complexity values. (#19384)

Author: lpaulino07

changelog.d/19384.doc

@@ -0,0 +1 @@
+Clarify how the estimated value of room complexity is calculated internally.

docs/usage/configuration/config_documentation.md

@@ -956,7 +956,7 @@ server_context: context
 ---
 ### `limit_remote_rooms`
 
-*(object)* When this option is enabled, the room "complexity" will be checked before a user joins a new remote room. If it is above the complexity limit, the server will disallow joining, or will instantly leave. This is useful for homeservers that are resource-constrained. Room complexity is an arbitrary measure based on factors such as the number of users in the room.
+*(object)* When this option is enabled, the room "complexity" will be checked before a user joins a new remote room. If it is above the complexity limit, the server will disallow joining, or will instantly leave. This is useful for homeservers that are resource-constrained. In Synapse, the complexity of a room is measured by the number of current state events in a room, divided by 500. "Current" here means the latest state, i.e. if a user joins, then leaves, then joins, that will count as 1 current `m.room.member` state event.
 
 This setting has the following sub-options:
 

schema/synapse-config.schema.yaml

@@ -1113,8 +1113,11 @@ properties:
       When this option is enabled, the room "complexity" will be checked before
       a user joins a new remote room. If it is above the complexity limit, the
       server will disallow joining, or will instantly leave. This is useful for
-      homeservers that are resource-constrained. Room complexity is an arbitrary
-      measure based on factors such as the number of users in the room.
+      homeservers that are resource-constrained. In Synapse, the complexity of a
+      room is measured by the number of current state events in a room, divided
+      by 500. "Current" here means the latest state, i.e. if a user joins, then
+      leaves, then joins, that will count as 1 current `m.room.member` state
+      event.
     properties:
       enabled:
         type: boolean