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/replication/logical/snapbuild.c

@@ -79,7 +79,7 @@
  *	  +--->|SNAPBUILD_CONSISTENT	 |<------------+
  *		   +-------------------------+
  *
- * Initially the machinery is in the START stage. When a xl_running_xacts
+ * Initially the machinery is in the START stage. When an xl_running_xacts
  * record is read that is sufficiently new (above the safe xmin horizon),
  * there's a state transition. If there were no running xacts when the
  * runnign_xacts record was generated, we'll directly go into CONSISTENT
@@ -161,7 +161,7 @@ struct SnapBuild
 
 	/*
 	 * Don't start decoding WAL until the "xl_running_xacts" information
-	 * indicates there are no running xids with a xid smaller than this.
+	 * indicates there are no running xids with an xid smaller than this.
 	 */
 	TransactionId initial_xmin_horizon;
 
@@ -683,8 +683,9 @@ SnapBuildProcessChange(SnapBuild *builder, TransactionId xid, XLogRecPtr lsn)
 }
 
 /*
- * Do CommandId/ComboCid handling after reading a xl_heap_new_cid record. This
- * implies that a transaction has done some form of write to system catalogs.
+ * Do CommandId/ComboCid handling after reading an xl_heap_new_cid record.
+ * This implies that a transaction has done some form of write to system
+ * catalogs.
  */
 void
 SnapBuildProcessNewCid(SnapBuild *builder, TransactionId xid,