Cross-link `matrix_synapse_forget_rooms_on_leave` and `matrix_synapse_forgotten_room_retention_period` variables

roles/custom/matrix-synapse/defaults/main.yml

@@ -1415,6 +1415,7 @@ matrix_synapse_default_room_version: "10"
 
 # Controls whether leaving a room will automatically forget it.
 # The upstream default is `false`, but we try to make Synapse less wasteful of resources, so we do things differently.
+# Also see: `matrix_synapse_forgotten_room_retention_period`
 matrix_synapse_forget_rooms_on_leave: true
 
 # Controls the Synapse `modules` list.
@@ -1455,6 +1456,7 @@ matrix_synapse_redaction_retention_period: 7d
 
 # Controls how long to keep locally forgotten rooms before purging them from the DB.
 # Defaults to `null`, meaning it's disabled.
+# Also see: `matrix_synapse_forget_rooms_on_leave`
 # Example value: 28d
 matrix_synapse_forgotten_room_retention_period: 28d