Improve the monotonic time documentation in compat_time.c
We discovered in #29500 (moved) that the monotonic time documentation is incomplete. Here are some improvements we could make:
- define "monotonic" as "strictly non-decreasing, delta t >= 0"
- explain when 0 time deltas are more likely:
- when measuring units larger than nanoseconds (due to truncation on division)
- on platforms with low resolution timers (because two time calls can happen faster than the resolution of the timer)
- on Windows, where the time is not always monotonic, due to an OS bug
- on platforms without specialised monotonic functions, during a wall clock time change
- explain what happens when the timers overflow?
Based on: https://gitweb.torproject.org/tor.git/tree/src/lib/time/compat_time.c#n175 https://trac.torproject.org/projects/tor/ticket/29500#comment:11
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