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/bufmgr.c

@@ -101,13 +101,13 @@ static volatile BufferDesc *PinCountWaitBuf = NULL;
  * Each buffer also has a private refcount that keeps track of the number of
  * times the buffer is pinned in the current process.  This is so that the
  * shared refcount needs to be modified only once if a buffer is pinned more
- * than once by a individual backend.  It's also used to check that no buffers
+ * than once by an individual backend.  It's also used to check that no buffers
  * are still pinned at the end of transactions and when exiting.
  *
  *
  * To avoid - as we used to - requiring an array with NBuffers entries to keep
  * track of local buffers, we use a small sequentially searched array
- * (PrivateRefCountArray) and a overflow hash table (PrivateRefCountHash) to
+ * (PrivateRefCountArray) and an overflow hash table (PrivateRefCountHash) to
  * keep track of backend local pins.
  *
  * Until no more than REFCOUNT_ARRAY_ENTRIES buffers are pinned at once, all