Consolidate receiver/filter/writer {pre,post}_select() docs.

Consolidate receiver/filter/writer {pre,post}_select() docs.

The documentation of these three ->pre_select() and ->post_select()
methods overlapped quite a bit. Some comments stated general properties
of the sched API which fit better in the documentation of sched.h,
so move these bits there.

Improve the text a bit while at it and avoid talking about select(2)
and fd sets as these are implementation details. Instead, focus on
the general concept of fd monitoring.

Pure comment cleanups, no code changes.