Update some incorrect comments about xlog records.

The comments claim that certain pieces of data are part of the main
WAL record data when in reality they are part of the data for
block 0. Repair.

Bertrand Drouvot, reviewed by Amit Kapila. Originally reported by me.

Discussion: http://postgr.es/m/80db7836-4415-d54a-64c3-66b88b1430e7@gmail.com

src/include/access/gistxlog.h

@@ -52,9 +52,7 @@ typedef struct gistxlogDelete
 	TransactionId snapshotConflictHorizon;
 	uint16		ntodelete;		/* number of deleted offsets */
 
-	/*
-	 * In payload of blk 0 : todelete OffsetNumbers
-	 */
+	/* TODELETE OFFSET NUMBER ARRAY FOLLOWS */
 } gistxlogDelete;
 
 #define SizeOfGistxlogDelete	(offsetof(gistxlogDelete, ntodelete) + sizeof(uint16))

src/include/access/heapam_xlog.h

@@ -345,8 +345,9 @@ typedef struct xl_heap_freeze_page
 	TransactionId snapshotConflictHorizon;
 	uint16		nplans;
 
-	/* FREEZE PLANS FOLLOW */
-	/* OFFSET NUMBER ARRAY FOLLOWS */
+	/*
+	 * In payload of blk 0 : FREEZE PLANS and OFFSET NUMBER ARRAY
+	 */
 } xl_heap_freeze_page;
 
 #define SizeOfHeapFreezePage (offsetof(xl_heap_freeze_page, nplans) + sizeof(uint16))

src/include/access/nbtxlog.h

@@ -236,9 +236,13 @@ typedef struct xl_btree_delete
 	uint16		ndeleted;
 	uint16		nupdated;
 
-	/* DELETED TARGET OFFSET NUMBERS FOLLOW */
-	/* UPDATED TARGET OFFSET NUMBERS FOLLOW */
-	/* UPDATED TUPLES METADATA (xl_btree_update) ARRAY FOLLOWS */
+	/*----
+	 * In payload of blk 0 :
+	 * - DELETED TARGET OFFSET NUMBERS
+	 * - UPDATED TARGET OFFSET NUMBERS
+	 * - UPDATED TUPLES METADATA (xl_btree_update) ARRAY
+	 *----
+	 */
 } xl_btree_delete;
 
 #define SizeOfBtreeDelete	(offsetof(xl_btree_delete, nupdated) + sizeof(uint16))