catalyst-spec.5.txt: Clarify "within the loop" (squashfs)

catalyst-spec.5.txt: Clarify "within the loop" (squashfs)

When you build a livecd-stage2, you get an ISO, inside of which is
something like:

  # mount -o loop livecd.iso iso
  # tree iso
  iso/
  |-- # stuff from livecd/overlay
  |-- image.squashfs  # the compressed root filesystem
  |-- isolinux
  |   |-- # kernel
  |   |-- # and
  |   `-- # related help
  |-- livecd          # empty, probably just a marker
  `-- README.txt      # help for kernel command line options

Inside the image.squashfs (or image.*, as configured by
livecd/fstype) is the root filesystem:

  # mount -o loop iso/image.squashfs squashfs
  # ls squashfs
  bin   dev  home  media  opt   root  sbin  tmp  var
  boot  etc  lib   mnt    proc  run   sys   usr

So if you want to tweak the root filesystem, use livecd/root_overlay.
If you want to tweak the ISO filesystem, use livecd/overlay.

Signed-off-by: W. Trevor King <wking@tremily.us>