f2fs: Add the 'whint_mode' mount option to f2fs documentation

Signed-off-by: Hyunchul Lee <cheol.lee@lge.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: Add the write-hint policy in f2fs doc.]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>