Doc: use "an SQL" consistently rather than "a SQL"

Doc: use "an SQL" consistently rather than "a SQL"

Similarly to what was done in 04539e73f, we standardized on SQL being
pronounced "es-que-ell" rather than "sequel" in our documentation.

Two inconsistencies have crept in during the v15 cycle.  The others
existed before but were missed in 04539e73f due to none of the searches
accounting for "SQL" being wrapped in tags.

As with 04539e73f, we don't touch code comments here in order to not
create unnecessary back-patching pain.

Discussion: https://postgr.es/m/CAApHDvpML27UqFXnrYO1MJddsKVMQoiZisPvsAGhKE_tsKXquw%40mail.gmail.com