iostats.txt: add easy-to-find description for field 6

iostats.txt has a merged description of fields 2 and 6, but it's not obvious.
This patch adds a field 6, pointing to the merged description of field 6 in
field 2.
Signed-off-by: David Hilton <david.hilton.p@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/iostats.txt

@@ -71,6 +71,8 @@ Field  4 -- # of milliseconds spent reading
     measured from __make_request() to end_that_request_last()).
 Field  5 -- # of writes completed
     This is the total number of writes completed successfully.
+Field  6 -- # of writes merged
+    See the description of field 2.
 Field  7 -- # of sectors written
     This is the total number of sectors written successfully.
 Field  8 -- # of milliseconds spent writing