There was much confusion over the functionality of the original
`TimerEvent::insert` call which was described as "Set relative timestamp
of the internal event".

This then extended to my Chrono conversion, meaning the new `insert`
call is not equivalent.

Clarify the original documentation, correct the deprecation messages,
and add more notes on conversion.

No functional change, as the new Chrono API makes more sense - it's just
different from the old API.

Problem actually spotted when I saw the strange code `convert_timestamp`
was producing for the 32-bit->64-bit timestamp conversion. The caller of
it was actually making the mistake of issuing
"TimerEvent::insert(rel_timeout)`, meaning they'd also misunderstood the
documentation, and were not getting the timeout they expected.

(Chrono would have prevented that mistake as durations and time points
are incompatible types).