storage: Better describe logical pool creation/definition parameters
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1398087
    
    Clean up the virsh man page description for --pool-create-as in order
    to better describe how the various arguments are used when creating
    (or defining) a logical pool.
    
    Signed-off-by: Ján Tomko <jtomko@redhat.com>