Add a description of pod states in kubernetes.

Closes #1146 
The point is that they are different, depending on the restart semantics.  In general, I can't say something like:

PodFailed:
if any container has exited with a non-zero status

Since in the restart cases, that may have happened, and you'll still be in PodRunning.  I think that splitting it out this way is actually the most clear way to do it.