autofs: update ioctl documentation regarding struct autofs_dev_ioctl

This is the same as bf72eda5 except that it's a different file.  Sync
documentation with changes made by 730c9eec in 2009.

Link: http://lkml.kernel.org/r/148577165630.9801.6081791213151121657.stgit@pluto.themaw.net


Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
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>