docs: Add callback-related info to virStream{Abort,Finish}

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>