timeout.9: document new interfaces, miscellaneous rewrites and cleanup

timeout.9: document new interfaces, miscellaneous rewrites and cleanup

- Document timeout_abs_ts(9).
- Add the kclock arguments to timeout_set_flags(9) and
  TIMEOUT_INITIALIZER_FLAGS(9).
- Document KCLOCK_NONE and KCLOCK_UPTIME.
- Mention the static initialization macros alongside timeout_set(9) etc.;
  keep relevant information adjacent.
- Mention timeout_add_sec(9) etc. alongside timeout_add(9); keep
  relevant information adjacent.

... plus many other cleanups, rewrites, and rearrangements.

Prompted by mvs@ and many others.  With input from jmc@, mvs@, kn@,
schwarze@, and probably a few others I have forgotten.

v1: https://marc.info/?l=openbsd-tech&m=162449274513068&w=2
v2: https://marc.info/?l=openbsd-tech&m=165851505627764&w=2
v3: https://marc.info/?l=openbsd-tech&m=167250339811308&w=2

ok jmc@ mvs@ schwarze@