Suggestion: add the ability to enter an "undrain" message when undraining nodes

[kkier]

In our workflow, current procedure when undraining a node after a repair is to do flux resource drain -ff foo101 "replaced flux capacitor see ticket bar51" and then flux resource undrain foo101 to return the node to service. To figure out what notes are attached to a node being undrained, we have to find the undrain time and then backtrack.

Instead, it would be useful to be able to add a message to a la flux resource undrain foo101 "replaced flux capacitor see ticket bar51" and then have that message available in the log coupled to the undrain event.