[zircon][kernel] Comment DPC contract

DPCs are a mechanism to queue work in the zircon kernel to run later from
thread context with interrupts enabled; comment the contract of DPCs more
completely.

* DPCs are run with interrupts enabled
* DPCs are queued to the current CPU and run there except in the presence of
  offlining/hotunplugging a CPU
* DPCs will never move CPUs while executing

Change-Id: Ib8d7d73cf6f8df99f49f632434387a3fd71f7f30