update documentation for UNIX_EPOCH and comparison between SystemTimes to clarify handling of failure

[david-d-h]

This documentation change is meant to clarify that SystemTimes may (platform-dependently) be "less" than the UNIX_EPOCH and a developer ought to take care in comparing between the two, as this might lead to an error. It also explicitly mentions SystemTimeError::duration for when such an error does occur.

I'm not sure on whether it would be helpful to document such a thing on the SystemTime itself rather than on it's methods and assoc consts? Perhaps it could be, since the documentation for the item also mentions the UNIX_EPOCH anchor and encourages comparisons with it. Anyways, open to suggestions.

closes #52522

[rustbot]

r? @joboet

rustbot has assigned @joboet.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer