Mention 'solo' and 'threads' pool implementations in celeryd CLI docs

celery/bin/celeryd.py

@@ -108,7 +108,7 @@ class WorkerCommand(Command):
                 default=conf.CELERYD_POOL,
                 action="store", dest="pool", type="str",
                 help="Pool implementation: "
-                     "processes (default), eventlet or gevent."),
+                     "processes (default), eventlet, gevent, solo or threads."),
             Option('--purge', '--discard', default=False,
                 action="store_true", dest="discard",
                 help="Discard all waiting tasks before the server is"