Fix inconsistencies and typos in the tree, take 11

This fixes various typos in docs and comments, and removes some orphaned
definitions.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/5da8e325-c665-da95-21e0-c8a99ea61fbf@gmail.com

src/include/access/brin_xlog.h

@@ -77,7 +77,7 @@ typedef struct xl_brin_insert
  * A cross-page update is the same as an insert, but also stores information
  * about the old tuple.
  *
- * Like in xlog_brin_update:
+ * Like in xl_brin_insert:
  * Backup block 0: new page, block data includes the new BrinTuple.
  * Backup block 1: revmap page
  *

src/include/access/heapam_xlog.h

@@ -168,7 +168,7 @@ typedef struct xl_heap_insert
  *
  * In block 0's data portion, there is an xl_multi_insert_tuple struct,
  * followed by the tuple data for each tuple. There is padding to align
- * each xl_multi_insert struct.
+ * each xl_multi_insert_tuple struct.
  */
 typedef struct xl_heap_multi_insert
 {

src/include/access/htup_details.h

@@ -233,7 +233,7 @@ struct HeapTupleHeaderData
 
 /*
  * A tuple that has HEAP_XMAX_IS_MULTI and HEAP_XMAX_LOCK_ONLY but neither of
- * XMAX_EXCL_LOCK and XMAX_KEYSHR_LOCK must come from a tuple that was
+ * HEAP_XMAX_EXCL_LOCK and HEAP_XMAX_KEYSHR_LOCK must come from a tuple that was
  * share-locked in 9.2 or earlier and then pg_upgrade'd.
  *
  * In 9.2 and prior, HEAP_XMAX_IS_MULTI was only set when there were multiple

src/include/access/xlog_internal.h

@@ -45,7 +45,7 @@ typedef struct XLogPageHeaderData
 	 * continue on the next page.  xlp_rem_len is the number of bytes
 	 * remaining from a previous page.
 	 *
-	 * Note that xl_rem_len includes backup-block data; that is, it tracks
+	 * Note that xlp_rem_len includes backup-block data; that is, it tracks
 	 * xl_tot_len not xl_len in the initial header.  Also note that the
 	 * continuation data isn't necessarily aligned.
 	 */