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 #1588

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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[djc]

Thanks!