pgsql: Doc: improve explanation of when custom GUCs appear in pg_settin

Doc: improve explanation of when custom GUCs appear in pg_settings.

Be more clear about when and how an extension-defined GUC comes to be
visible in pg_settings. (Move the para to the bottom of the page, too;
whoever thought this point was more important than the para about the
view being updatable had odd priorities IMNSHO.)

Back-patch to v15 where archive modules were added, since that seems
to have made this more of a sore spot than it was before.

Benoit Lobréau, Nathan Bossart

Discussion: https://postgr.es/m/CAPE8EZ7KHaXMHKwT=HOim23tDVKYA1PruRuTfeYdCrYWwPGhag@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/44a51dc990cc39fcd5ae6359bf1e27bd6699449b

Modified Files
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doc/src/sgml/system-views.sgml | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)