docs: clarify database_host ignored when postgres sockets are enabled

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

@@ -1310,6 +1310,10 @@ matrix_synapse_sentry_dsn: ""
 
 # Postgres database information
 matrix_synapse_database_txn_limit: 0
+#
+# Use this hostname for TCP-based Postgres connections.
+# When `matrix_synapse_database_socket_enabled` is true, this is ignored and
+# `matrix_synapse_database_socket_path` is used instead.
 matrix_synapse_database_host: ''
 matrix_synapse_database_port: 5432
 matrix_synapse_database_cp_min: 5