Do not let people enable more than 1 federation_sender worker

roles/matrix-synapse/defaults/main.yml

@@ -320,6 +320,9 @@ matrix_synapse_workers_pusher_workers_metrics_range_start: 19200
 matrix_synapse_workers_appservice_workers_count: 1
 matrix_synapse_workers_appservice_workers_metrics_range_start: 19300
 
+# matrix_synapse_workers_federation_sender_workers_count can only be 0 or 1 for now.
+# More instances are not supported due to a playbook limitation having to do with keeping `federation_sender_instances` in `homeserver.yaml` updated.
+# See https://github.com/matrix-org/synapse/blob/master/docs/workers.md#synapseappfederation_sender
 matrix_synapse_workers_federation_sender_workers_count: 1
 matrix_synapse_workers_federation_sender_workers_metrics_range_start: 19400
 

roles/matrix-synapse/tasks/validate_config.yml

@@ -20,6 +20,7 @@
   with_items:
     - "matrix_synapse_workers_appservice_workers_count"
     - "matrix_synapse_workers_pusher_workers_count"
+    - "matrix_synapse_workers_federation_sender_workers_count"
 
 - name: (Deprecation) Catch and report renamed settings
   fail: