Correct several behavior descriptions in comments.

Reuse cautionary language from src/test/ssl/README in
src/test/kerberos/README.  SLRUs have had access to six-character
segments names since commit 73c986adde,
and recovery stopped calling HeapTupleHeaderAdvanceLatestRemovedXid() in
commit 558a9165e0.  The other corrections
are more self-evident.

src/backend/access/heap/heapam.c

@@ -6920,8 +6920,6 @@ HeapTupleHeaderAdvanceLatestRemovedXid(HeapTupleHeader tuple,
 	 * updated/deleted by the inserting transaction.
 	 *
 	 * Look for a committed hint bit, or if no xmin bit is set, check clog.
-	 * This needs to work on both primary and standby, where it is used to
-	 * assess btree delete records.
 	 */
 	if (HeapTupleHeaderXminCommitted(tuple) ||
 		(!HeapTupleHeaderXminInvalid(tuple) && TransactionIdDidCommit(xmin)))