instance_states: explain arrows between {moving, moved} and dying -> dead

[Minoru]

Looking at the diagram again, it's not quite clear why an instance would change from "moving" to "moved" and back. The answer is that "moving" becomes "moved" after a few redirects, and "moved" becomes "moving" if the redirection target changes.

Similarly, it's unclear why a "dying" node would become a "dead" node. The answer is again the number of failed requests.

It might also make sense to colour the boxes the same way as the arrows, so it's easier to see which box describes which kind of arrows.