native/date: Improve DelayedFormat doc re Panics

[behnam-oneschema]

Since #[should_panic] kind of checks are not available for doctesting, cannot add a test for it in the current structure. The panic behavior probably could use a unit test somewhere else, if it doesn't exist yet.

See https://github.com/chronotope/chrono/issues/1588

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.78%. Comparing base (2085627) to head (c299c18).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1590 +/- ## ======================================= Coverage 91.78% 91.78% ======================================= Files 37 37 Lines 18208 18208 ======================================= Hits 16713 16713 Misses 1495 1495 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.