Collection of typo fixes.

Use "a" and "an" correctly, mostly in comments. Two error messages were
also fixed (they were just elogs, so no translation work required). Two
function comments in pg_proc.h were also fixed. Etsuro Fujita reported one
of these, but I found a lot more with grep.

Also fix a few other typos spotted while grepping for the a/an typos.
For example, "consists out of ..." -> "consists of ...". Plus a "though"/
"through" mixup reported by Euler Taveira.

Many of these typos were in old code, which would be nice to backpatch to
make future backpatching easier. But much of the code was new, and I didn't
feel like crafting separate patches for each branch. So no backpatching.

contrib/test_decoding/sql/decoding_in_xact.sql

@@ -7,7 +7,7 @@ SELECT txid_current() = 0;
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
 ROLLBACK;
 
--- fail because we're creating a slot while in an subxact whose topxact has a xid
+-- fail because we're creating a slot while in a subxact whose topxact has an xid
 BEGIN;
 SELECT txid_current() = 0;
 SAVEPOINT barf;