Document that Synapse pusher worker instances are shardable

Related to:
- https://github.com/matrix-org/synapse/pull/9407
- https://github.com/matrix-org/synapse/pull/7855

roles/matrix-synapse/defaults/main.yml

@@ -312,7 +312,9 @@ matrix_synapse_workers_generic_workers_count: 3
 matrix_synapse_workers_generic_workers_port_range_start: 18111
 matrix_synapse_workers_generic_workers_metrics_range_start: 19111
 
-# matrix_synapse_workers_pusher_workers_count can only be 0 or 1. More instances are not supported.
+# matrix_synapse_workers_pusher_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 `pusher_instances` in `homeserver.yaml` updated.
+# See https://github.com/matrix-org/synapse/commit/ddfdf945064925eba761ae3748e38f3a1c73c328
 matrix_synapse_workers_pusher_workers_count: 1
 matrix_synapse_workers_pusher_workers_metrics_range_start: 19200