sbin/mount_autofs: Add description

sbin/mount_autofs: Add description

TL;DR users won't be running this command to mount autofs.

Having autofs_enable=YES won't run this command either.
There is a high level command automount(8) and that does everything
based on /etc/auto_master contents.

mount_autofs(8) is a low level version where users can manually
specify things automount(8) internally passes to mount(2).
Thus it's not easy or needed to understand what -fOp options exactly
do without having clear understanding of autofs internal.
But I'll write it here in case interested.

 -f is a string which eventually gets copied to statfs::f_mntfromname.
  Unlike most of the filesystems with backing store, this is just
  a label to identify the autofs mount.
 -O is mount options of autofs(5).
 -p is a prefix string to distinct direct and indirect mapping.