Fix inconsistencies and typos in the tree

This is numbered take 7, and addresses a set of issues around:
- Fixes for typos and incorrect reference names.
- Removal of unneeded comments.
- Removal of unreferenced functions and structures.
- Fixes regarding variable name consistency.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/10bfd4ac-3e7c-40ab-2b2e-355ed15495e8@gmail.com

src/backend/access/gin/README

@@ -163,7 +163,7 @@ algorithms.
 
 * The posting list can be accessed with GinGetPosting(itup)
 
-* If GinITupIsCompressed(itup), the posting list is stored in compressed
+* If GinItupIsCompressed(itup), the posting list is stored in compressed
   format. Otherwise it is just an array of ItemPointers. New tuples are always
   stored in compressed format, uncompressed items can be present if the
   database was migrated from 9.3 or earlier version.

src/backend/access/gin/ginfast.c

@@ -1013,7 +1013,7 @@ ginInsertCleanup(GinState *ginstate, bool full_clean,
 
 	/*
 	 * As pending list pages can have a high churn rate, it is desirable to
-	 * recycle them immediately to the FreeSpace Map when ordinary backends
+	 * recycle them immediately to the FreeSpaceMap when ordinary backends
 	 * clean the list.
 	 */
 	if (fsm_vac && fill_fsm)

src/backend/access/gist/README

@@ -170,7 +170,7 @@ it splits the page, and constructs the new downlink tuples for the split
 pages. The caller must then call gistplacetopage() on the parent page to
 insert the downlink tuples. The parent page that holds the downlink to
 the child might have migrated as a result of concurrent splits of the
-parent, gistfindCorrectParent() is used to find the parent page.
+parent, gistFindCorrectParent() is used to find the parent page.
 
 Splitting the root page works slightly differently. At root split,
 gistplacetopage() allocates the new child pages and replaces the old root

src/backend/access/gist/gist.c

@@ -821,7 +821,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 			/*
 			 * Leaf page. Insert the new key. We've already updated all the
 			 * parents on the way down, but we might have to split the page if
-			 * it doesn't fit. gistinserthere() will take care of that.
+			 * it doesn't fit. gistinserttuple() will take care of that.
 			 */
 
 			/*

src/backend/access/nbtree/README

@@ -457,7 +457,7 @@ right sibling's left-link --- followed by a second WAL entry for the
 insertion on the parent level (which might itself be a page split, requiring
 an additional insertion above that, etc).
 
-For a root split, the followon WAL entry is a "new root" entry rather than
+For a root split, the follow-on WAL entry is a "new root" entry rather than
 an "insertion" entry, but details are otherwise much the same.
 
 Because splitting involves multiple atomic actions, it's possible that the

src/backend/access/spgist/spgscan.c

@@ -146,11 +146,6 @@ resetSpGistScanOpaque(SpGistScanOpaque so)
 {
 	MemoryContext oldCtx;
 
-	/*
-	 * clear traversal context before proceeding to the next scan; this must
-	 * not happen before the freeScanStack above, else we get double-free
-	 * crashes.
-	 */
 	MemoryContextReset(so->traversalCxt);
 
 	oldCtx = MemoryContextSwitchTo(so->traversalCxt);

src/backend/access/transam/clog.c

@@ -155,7 +155,7 @@ static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
  * NB: this is a low-level routine and is NOT the preferred entry point
  * for most uses; functions in transam.c are the intended callers.
  *
- * XXX Think about issuing FADVISE_WILLNEED on pages that we will need,
+ * XXX Think about issuing POSIX_FADV_WILLNEED on pages that we will need,
  * but aren't yet in cache, as well as hinting pages not to fall out of
  * cache yet.
  */