sched: Add a comment to effective_load() since it's a pain

Every time I have to stare at this function I need to completely
reverse engineer its workings, about time I write a comment
explaining the thing.

Collected bits and pieces from previous changelogs, mostly:

  4be9daaa
  83378269



Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1318518057.27731.2.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>