ualarm.3: cleanups, rewrites

ualarm.3: cleanups, rewrites

Clean up the ualarm.3 page as we did with the alarm.3 page.

- Be more specific in the Name summary.
- Rewrite the Description.  In particular, ualarm(3) does not block,
  it schedules the signal for asynchronous delivery.
- The Return Values section is incoherent, rewrite it.  Remove the
  incorrect details about the upper bound.
- Add Caveats: discourage use of ualarm(3) and setitimer(2) in the
  same program.

Input on v1 from schwarze@, deraadt@, millert@, and jmc@.  Input on v2
from jmc@ and millert@.

v1: https://marc.info/?l=openbsd-tech&m=162508423008134&w=2
v2: https://marc.info/?l=openbsd-tech&m=169076642509763&w=2

ok jmc@ millert@