Use the correct article for abbreviations

We've accumulated quite a mix of instances of "an SQL" and "a SQL" in the
documents.  It would be good to be a bit more consistent with these.

The most recent version of the SQL standard I looked at seems to prefer
"an SQL".  That seems like a good lead to follow, so here we change all
instances of "a SQL" to become "an SQL".  Most instances correctly use
"an SQL" already, so it also makes sense to use the dominant variation in
order to minimise churn.

Additionally, there were some other abbreviations that needed to be
adjusted. FSM, SSPI, SRF and a few others.  Also fix some pronounceable,
abbreviations to use "a" instead of "an".  For example, "a SASL" instead
of "an SASL".

Here I've only adjusted the documents and error messages.  Many others
still exist in source code comments.  Translator hint comments seem to be
the biggest culprit.  It currently does not seem worth the churn to change
these.

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

doc/src/sgml/ref/create_trigger.sgml

@@ -515,7 +515,7 @@ UPDATE OF <replaceable>column_name1</replaceable> [, <replaceable>column_name2</
    as <literal>ON UPDATE CASCADE</literal> or <literal>ON DELETE SET NULL</literal>, are
    treated as part of the SQL command that caused them (note that such
    actions are never deferred).  Relevant triggers on the affected table will
-   be fired, so that this provides another way in which a SQL command might
+   be fired, so that this provides another way in which an SQL command might
    fire triggers not directly matching its type.  In simple cases, triggers
    that request transition relations will see all changes caused in their
    table by a single original SQL command as a single transition relation.