docs: Add callback-related info to virStream{Abort,Finish}
    
    When one has a non-blocking stream and aborts or finishes it without
    removing the callback, any event loop invocation will trigger that
    callback, but it cannot be removed any more.  We cannot remove the
    callback automatically from virStream{Abort,Finish} functions due to
    forward-compatibility.  So let's at least document this behaviour,
    because it is not easy to find out the reason for.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>