Fix nbtree page split rmgr desc routine.

Include newitemoff in rmgr desc output for nbtree page split records.
In passing, correct an obsolete comment that claimed that newitemoff is
only logged for _L variant nbtree page split WAL records.

Both issues were oversights in commit 2c03216d, which revamped the
WAL format.

Author: Peter Geoghegan
Backpatch: 9.5-, where the WAL format was revamped.

src/backend/access/rmgrdesc/nbtdesc.c

@@ -38,8 +38,8 @@ btree_desc(StringInfo buf, XLogReaderState *record)
 			{
 				xl_btree_split *xlrec = (xl_btree_split *) rec;
 
-				appendStringInfo(buf, "level %u, firstright %d",
-								 xlrec->level, xlrec->firstright);
+				appendStringInfo(buf, "level %u, firstright %d, newitemoff %d",
+								 xlrec->level, xlrec->firstright, xlrec->newitemoff);
 				break;
 			}
 		case XLOG_BTREE_VACUUM:

src/include/access/nbtxlog.h

@@ -82,13 +82,12 @@ typedef struct xl_btree_insert
  *
  * Note: XLOG_BTREE_SPLIT_L and XLOG_BTREE_SPLIT_R share this data record.
  * There are two variants to indicate whether the inserted tuple went into the
- * left or right split page (and thus, whether newitemoff and the new item are
- * stored or not).  We always log the left page high key because suffix
- * truncation can generate a new leaf high key using user-defined code.  This
- * is also necessary on internal pages, since the first right item that the
- * left page's high key was based on will have been truncated to zero
- * attributes in the right page (the original is unavailable from the right
- * page).
+ * left or right split page (and thus, whether the new item is stored or not).
+ * We always log the left page high key because suffix truncation can generate
+ * a new leaf high key using user-defined code.  This is also necessary on
+ * internal pages, since the first right item that the left page's high key
+ * was based on will have been truncated to zero attributes in the right page
+ * (the original is unavailable from the right page).
  *
  * Backup Blk 0: original page / new left page
  *
@@ -112,7 +111,7 @@ typedef struct xl_btree_split
 {
 	uint32		level;			/* tree level of page being split */
 	OffsetNumber firstright;	/* first item moved to right page */
-	OffsetNumber newitemoff;	/* new item's offset (if placed on left page) */
+	OffsetNumber newitemoff;	/* new item's offset (useful for _L variant) */
 } xl_btree_split;
 
 #define SizeOfBtreeSplit	(offsetof(xl_btree_split, newitemoff) + sizeof(OffsetNumber))