We discovered in legacy/trac#29500 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