Improve comment about query_id_enabled in queryjumblefuncs.c

The comment was inexact because query_id_enabled will not be switched to
"true" even if compute_query_id is "on", unless a module requests for
it.

While on it, this adds a comment to mention that IsQueryIdEnabled()
should be used to check if query ID computation is enabled or not.

Author: Yugo Nagata
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/20240209153823.e29a68cadb14225f1362a2cf@sraoss.co.jp

src/backend/nodes/queryjumblefuncs.c

@@ -42,7 +42,13 @@
 /* GUC parameters */
 int			compute_query_id = COMPUTE_QUERY_ID_AUTO;
 
-/* True when compute_query_id is ON, or AUTO and a module requests them */
+/*
+ * True when compute_query_id is ON or AUTO, and a module requests them.
+ *
+ * Note that IsQueryIdEnabled() should be used instead of checking
+ * query_id_enabled or compute_query_id directly when we want to know
+ * whether query identifiers are computed in the core or not.
+ */
 bool		query_id_enabled = false;
 
 static void AppendJumble(JumbleState *jstate,