Document format_distance (#3214)

Adds docs for `ui2::util::format_distance`:
- distance_in_seconds
- distance_string
- naive_format_distance
- naive_format_distance_from_now

These are ports of
[`date-fns`](https://date-fns.org/v2.30.0/docs/formatDistance)'s format
distance utilities

[[PR Description]]

Release Notes:

- N/A