Document and clean up gistsplit.c.

Improve comments, rename some variables and functions, slightly simplify
a couple of APIs, in an attempt to make this code readable by people other
than its original author.

Even though this is essentially just cosmetic, back-patch to all active
branches, because otherwise it's going to make back-patching future fixes
in this file very painful.

Author: tglsfdc

src/backend/access/gist/gist.c

@@ -1237,18 +1237,12 @@ gistSplit(Relation r,
 	IndexTuple *lvectup,
 			   *rvectup;
 	GistSplitVector v;
-	GistEntryVector *entryvec;
 	int			i;
 	SplitedPageLayout *res = NULL;
 
-	/* generate the item array */
-	entryvec = palloc(GEVHDRSZ + (len + 1) * sizeof(GISTENTRY));
-	entryvec->n = len + 1;
-
 	memset(v.spl_lisnull, TRUE, sizeof(bool) * giststate->tupdesc->natts);
 	memset(v.spl_risnull, TRUE, sizeof(bool) * giststate->tupdesc->natts);
-	gistSplitByKey(r, page, itup, len, giststate,
-				   &v, entryvec, 0);
+	gistSplitByKey(r, page, itup, len, giststate, &v, 0);
 
 	/* form left and right vector */
 	lvectup = (IndexTuple *) palloc(sizeof(IndexTuple) * (len + 1));