docs: Clarification for when allowed to use 'lun' for "volume"
    
    While re-reading what I wrote for commit id '785a8940e', I realized
    I needed to clarify that being able to present as a 'lun', the mode
    property for the pool source element needed to be "host" (or empty)
    and not "direct".
    
    It was described correctly later in the mode host description, but
    this just ensures it's not missed here as well.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>