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
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