Document the change from the old "mode" parameter to the "mode_option"
parameter.

Signed-off-by: Mike Pagano <mpagano@gentoo.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>