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

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