Add a comment describing how pagedaemon_wakeup() should be used and

synchronized.

Suggested by:	tegge

sys/vm/vm_pageout.c

@@ -1481,6 +1481,12 @@ vm_pageout()
 	}
 }
 
+/*
+ * Unless the page queue lock is held by the caller, this function
+ * should be regarded as advisory.  Specifically, the caller should
+ * not msleep() on &cnt.v_free_count following this function unless
+ * the page queue lock is held until the msleep() is performed.
+ */
 void
 pagedaemon_wakeup()
 {