autofs: update AUTOFS_EXP_LEAVES description

Update the description of AUTOFS_EXP_LEAVES to cover its possible future
use with amd format mount maps.

Link: http://lkml.kernel.org/r/155287083538.12593.18163159677020718048.stgit@pluto.themaw.net


Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>