nilfs2: add description of NILFS_IOCTL_SET_SUINFO ioctl

Add description of NILFS_IOCTL_SET_SUINFO ioctl in
Documentation/filesystems/nilfs2.txt to make it up-to-date.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Andreas Rohner <andreas.rohner@gmx.net>