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/include/access/xlog_internal.h

@@ -43,11 +43,8 @@ typedef struct XLogPageHeaderData
 	/*
 	 * When there is not enough space on current page for whole record, we
 	 * continue on the next page.  xlp_rem_len is the number of bytes
-	 * remaining from a previous page.
-	 *
-	 * Note that xlp_rem_len includes backup-block data; that is, it tracks
-	 * xl_tot_len not xl_len in the initial header.  Also note that the
-	 * continuation data isn't necessarily aligned.
+	 * remaining from a previous page; it tracks xl_tot_len in the initial
+	 * header.  Note that the continuation data isn't necessarily aligned.
 	 */
 	uint32		xlp_rem_len;	/* total len of remaining data for record */
 } XLogPageHeaderData;