Make rocksdb.rocksdb_range pass: MariaDB doesnt support ICP over reverse index scans atm

storage/rocksdb/mysql-test/rocksdb/r/rocksdb_range.result

@@ -198,10 +198,13 @@ max(pk)
 # #48: index_read_map(HA_READ_PREFIX_LAST) does not work in reverse CF
 #
 # Tests for search_flag=HA_READ_PREFIX_LAST_OR_PREV
+# Note: the next explain has "Using index condition" in fb/mysql-5.6
+#       but "Using where" in MariaDB because the latter does not 
+#       support ICP over reverse scans.
 explain
 select * from t2 where a between 99 and 2000 order by a desc;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	range	a	a	4	NULL	#	Using index condition
+1	SIMPLE	t2	range	a	a	4	NULL	#	Using where
 select * from t2 where a between 99 and 2000 order by a desc;
 pk	a	b
 999	99	999

storage/rocksdb/mysql-test/rocksdb/t/rocksdb_range.test

@@ -128,6 +128,9 @@ select max(pk) from t3 where a=3 and pk < 33;
 --echo #
 
 --echo # Tests for search_flag=HA_READ_PREFIX_LAST_OR_PREV
+--echo # Note: the next explain has "Using index condition" in fb/mysql-5.6
+--echo #       but "Using where" in MariaDB because the latter does not 
+--echo #       support ICP over reverse scans.
 --replace_column 9 #
 explain
 select * from t2 where a between 99 and 2000 order by a desc;