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/optimizer/README

@@ -656,7 +656,7 @@ EquivalenceClasses and PathKeys after query_planner has completed.
 In these cases, the extra ECs/PKs are needed to represent sort orders
 that were not considered during query_planner.  Such situations should be
 minimized since it is impossible for query_planner to return a plan
-producing such a sort order, meaning a explicit sort will always be needed.
+producing such a sort order, meaning an explicit sort will always be needed.
 Currently this happens only for queries involving multiple window functions
 with different orderings, for which extra sorts are needed anyway.
 

src/backend/optimizer/path/indxpath.c

@@ -3968,7 +3968,7 @@ prefix_quals(Node *leftop, Oid opfamily, Oid collation,
 }
 
 /*
- * Given a leftop and a rightop, and a inet-family sup/sub operator,
+ * Given a leftop and a rightop, and an inet-family sup/sub operator,
  * generate suitable indexqual condition(s).  expr_op is the original
  * operator, and opfamily is the index opfamily.
  */