[docs] revoke+terminate only supported by prefork

docs/userguide/workers.rst

@@ -307,7 +307,7 @@ Commands
 
 ``revoke``: Revoking tasks
 --------------------------
-:pool support: all
+:pool support: all, terminate only supported by prefork
 :broker support: *amqp, redis*
 :command: :program:`celery -A proj control revoke <task_id>`