Fix misuse of table_index_fetch_tuple_check().

Commit 0d861bbb, which added deduplication to nbtree, had
_bt_check_unique() pass a TID to table_index_fetch_tuple_check() that
isn't safe to mutate.  table_index_fetch_tuple_check()'s tid argument is
modified when the TID in question is not the latest visible tuple in a
hot chain, though this wasn't documented.

To fix, go back to using a local copy of the TID in _bt_check_unique(),
and update comments above table_index_fetch_tuple_check().

Backpatch: 13-, where B-Tree deduplication was introduced.

src/backend/access/nbtree/nbtinsert.c

@@ -597,7 +597,8 @@ _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
 					 * not part of this chain because it had a different index
 					 * entry.
 					 */
-					if (table_index_fetch_tuple_check(heapRel, &itup->t_tid,
+					htid = itup->t_tid;
+					if (table_index_fetch_tuple_check(heapRel, &htid,
 													  SnapshotSelf, NULL))
 					{
 						/* Normal case --- it's still live */

src/backend/access/table/tableam.c

@@ -196,6 +196,10 @@ table_beginscan_parallel(Relation relation, ParallelTableScanDesc parallel_scan)
  * optimized, but is unlikely to matter from a performance POV. If there
  * frequently are live index pointers also matching a unique index key, the
  * CPU overhead of this routine is unlikely to matter.
+ *
+ * Note that *tid may be modified when we return true if the AM supports
+ * storing multiple row versions reachable via a single index entry (like
+ * heap's HOT).
  */
 bool
 table_index_fetch_tuple_check(Relation rel,