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.

src/backend/access/transam/commit_ts.c

@@ -122,7 +122,7 @@ static void WriteSetTimestampXlogRec(TransactionId mainxid, int nsubxids,
  * subtrans implementation changes in the future, we might want to revisit the
  * decision of storing timestamp info for each subxid.
  *
- * The do_xlog parameter tells us whether to include a XLog record of this
+ * The do_xlog parameter tells us whether to include an XLog record of this
  * or not.  Normal path through RecordTransactionCommit() will be related
  * to a transaction commit XLog record, and so should pass "false" here.
  * Other callers probably want to pass true, so that the given values persist