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/buf_init.c

@@ -50,7 +50,7 @@ char	   *BufferBlocks;
  * refcount --	Counts the number of processes holding pins on a buffer.
  *		A buffer is pinned during IO and immediately after a BufferAlloc().
  *		Pins must be released before end of transaction.  For efficiency the
- *		shared refcount isn't increased if a individual backend pins a buffer
+ *		shared refcount isn't increased if an individual backend pins a buffer
  *		multiple times. Check the PrivateRefCount infrastructure in bufmgr.c.
  */
 

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

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);