DOC: design: update the task vs thread affinity requirements

DOC: design: update the task vs thread affinity requirements

It looks like we'll need:
  - one share timers queue for the rare tasks that need to wake up
    anywhere
  - one private timers queue per thread
  - one global queue per group
  - one local queue per thread

And may be we can simply get rid of any global/shared run queue as
we don't seem to have any task bound to a subset of threads.