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/storage/buffer/freelist.c

@@ -149,7 +149,7 @@ ClockSweepTick(void)
 				 * allows other readers to read nextVictimBuffer and
 				 * completePasses in a consistent manner which is required for
 				 * StrategySyncStart().  In theory delaying the increment
-				 * could lead to a overflow of nextVictimBuffers, but that's
+				 * could lead to an overflow of nextVictimBuffers, but that's
 				 * highly unlikely and wouldn't be particularly harmful.
 				 */
 				SpinLockAcquire(&StrategyControl->buffer_strategy_lock);