fat: document additional vfat mount options

While debugging a sync mount regression on vfat I noticed that there were
mount options parsed by the driver that were not documented.

[hirofumi@mail.parknet.co.jp: fix some parts]
Signed-off-by: Bart Trojanowski <bart@jukie.net>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>