Document rounding in std::time::Duration's subsec_millis etc.

[sunfishcode]

In std::time::Duration's documentation, it's stated how as_secs rounds, however it's not stated how sunsec_millis and subsec_micros round.

In the implementations:

pub const fn subsec_millis(&self) -> u32 { self.nanos / NANOS_PER_MILLI }

they round toward zero.

[fintelia]

The same change should also probably be made to the (currently unstable) Duration::as_millis and Duration::as_micros methods. They round the same way.