Improve description of some WAL records for GIN

The following information is added in the description of some GIN
records:
- In INSERT_LISTPAGE, the number of tuples and the right link block.
- In UPDATE_META_PAGE, the number of tuples, the previous tail block,
and the right link block.
- In SPLIT, the left and right children blocks.

Author: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CALdSSPgnAt5L=D_xGXRXLYO5FK1H31_eYEESxdU1n-r4g+6GqA@mail.gmail.com

src/backend/access/rmgrdesc/gindesc.c

@@ -130,6 +130,9 @@ gin_desc(StringInfo buf, XLogReaderState *record)
 				appendStringInfo(buf, " isdata: %c isleaf: %c",
 								 (xlrec->flags & GIN_INSERT_ISDATA) ? 'T' : 'F',
 								 (xlrec->flags & GIN_INSERT_ISLEAF) ? 'T' : 'F');
+				if (xlrec->leftChildBlkno != InvalidBlockNumber)
+					appendStringInfo(buf, " children: %u/%u",
+									 xlrec->leftChildBlkno, xlrec->rightChildBlkno);
 			}
 			break;
 		case XLOG_GIN_VACUUM_PAGE:
@@ -150,10 +153,27 @@ gin_desc(StringInfo buf, XLogReaderState *record)
 			/* no further information */
 			break;
 		case XLOG_GIN_UPDATE_META_PAGE:
-			/* no further information */
+			{
+				ginxlogUpdateMeta *xlrec = (ginxlogUpdateMeta *) rec;
+
+				appendStringInfo(buf, "ntuples: %d", xlrec->ntuples);
+				if (xlrec->prevTail != InvalidBlockNumber)
+					appendStringInfo(buf, " prevTail: %u",
+									 xlrec->prevTail);
+				if (xlrec->newRightlink != InvalidBlockNumber)
+					appendStringInfo(buf, " newRightLink: %u",
+									 xlrec->newRightlink);
+			}
 			break;
 		case XLOG_GIN_INSERT_LISTPAGE:
-			/* no further information */
+			{
+				ginxlogInsertListPage *xlrec = (ginxlogInsertListPage *) rec;
+
+				appendStringInfo(buf, "ntuples: %d", xlrec->ntuples);
+				if (xlrec->rightlink != InvalidBlockNumber)
+					appendStringInfo(buf, " rightlink: %u",
+									 xlrec->rightlink);
+			}
 			break;
 		case XLOG_GIN_DELETE_LISTPAGE:
 			appendStringInfo(buf, "ndeleted: %d",