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>