autofs: fix Documentation regarding devid on ioctl

The explanation on how ioctl handles devid seems incorrect.  Userspace who
calls this ioctl has no input regarding devid, and ioctl implementation
retrieves devid via superblock.

Link: http://lkml.kernel.org/r/20160812024825.12352.13486.stgit@pluto.themaw.net


Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Ian Kent <ikent@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>