kbase: Document QEMU private mount NS limitations

kbase: Document QEMU private mount NS limitations

There are two points I've taken for granted:

  1) the mount points are set before starting a guest,
  2) the / and its submounts are marked as shared, so that mount
     events propagate into child namespaces when assumption 1) is
     not held.

But what's obvious to me might not be obvious to our users.
Document these known limitations.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2123196
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>