md.txt has two sections describing the 'level' sysfs attribute, and some of
the text is out-of-date.  So make just one section, and make it right.

Cc: Christian Kujau <evil@g-house.de>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>