Update comments to match new debug_sync implementation

mysql-test/main/debug_sync.test

@@ -231,15 +231,12 @@ SHOW VARIABLES LIKE 'DEBUG_SYNC';
 #       immediately after setting of the DEBUG_SYNC variable.
 #       So it is executed before the SET statement ends.
 #
-# NOTE: There is only one global signal (say "signal post" or "flag mast").
-#       A SIGNAL action writes its signal into it ("sets a flag").
-#       The signal persists until explicitly overwritten.
+# NOTE: There can be multiple active signals at the same time.
+#       A SIGNAL action appends its signal into signals set.
+#       The signal persists until waited on.
 #       To avoid confusion for later tests, it is recommended to clear
-#       the signal by signalling "empty" ("setting the 'empty' flag"):
-#       SET DEBUG_SYNC= 'now SIGNAL empty';
-#       Preferably you can reset the whole facility with:
+#       the signal set by running
 #       SET DEBUG_SYNC= 'RESET';
-#       The signal is then '' (really empty) which connot be done otherwise.
 #
 
 #

sql/debug_sync.cc

@@ -1344,7 +1344,8 @@ static bool debug_sync_eval_action(THD *thd, char *action_str, char *action_end)
   /*
     Try NO_CLEAR_EVENT.
   */
-  if (!my_strcasecmp(system_charset_info, token, "NO_CLEAR_EVENT")) {
+  if (!my_strcasecmp(system_charset_info, token, "NO_CLEAR_EVENT"))
+  {
     action->clear_event= false;
     /* Get next token. If none follows, set action. */
     if (!(ptr = debug_sync_token(&token, &token_length, ptr, action_end))) goto set_action;
@@ -1634,8 +1635,8 @@ static void debug_sync_execute(THD *thd, st_debug_sync_action *action)
 
 
       /*
-        Wait until global signal string matches the wait_for string.
-        Interrupt when thread or query is killed or facility disabled.
+        Wait until the signal set contains the wait_for string.
+        Interrupt when thread or query is killed or facility is disabled.
         The facility can become disabled when some thread cannot get
         the required dynamic memory allocated.
       */