RustDocs for SystemTime and Instant should note that their Add<Duration> implementations panic

[woody77]

This note: https://doc.rust-lang.org/std/time/struct.SystemTime.html#panics is too deeply buried in the documentation.

Proposal:

Put a note after the underlying syscalls section in the overall struct documentation that math operations on SystemTime (and Instant) can panic.

[poliorcetics]

@rustbot claim