Improve the monotonic time documentation in compat_time.c

We discovered in legacy/trac#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?