pgsql: Improve comment about query_id_enabled in queryjumblefuncs.c

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f854dae888ab69db92153369644951dfb5b3d034

Modified Files
--------------
src/backend/nodes/queryjumblefuncs.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)