docs: Improve documentation of <disk type='dir'>

Note the implications and caveats of <disk type='dir'>.

Closes: #519


Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>