SystemTime docs: recommend Instant for elapsed time
Introduction to `SystemTime` mentions problems with non-monotonic clocks, but individual methods don't.
For benefit of users who jump directly to method's documentation, also recommend `Instant` in `elapsed` and `duration_since`.
`SystemTime::elapsed()` docs overpromised the elapsed time. It's not elapsed time, but a difference between two clocks.
Mazdak Farrokhzad
117fa1de98