Add documentation tooltip to metric graphs

[bryk]

Frequently I hear users complaining that the metric values we display are incorrect. But in fact, they didn't know what the value was. This is obviously our fault, because we do not explain the problem.

How about we add, e.g., a documentation bubble explaining what the metrics there are?

Example problem: kubernetes-retired/heapster#1356

[bryk]

Update: there's more people complaining that graphs on node views include cache, which is super misleading.