Editorial: Add explanatory note to CalendarMonthDayFromFields

[catamorphism]

Explain why the ISODateWithinLimits check is necessary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (4197642) to head (57e18ca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3192   +/-   ##
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  Coverage   96.67%   96.67%           
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  Files          22       22           
  Lines       10357    10357           
  Branches     1859     1859           
=======================================
  Hits        10013    10013           
  Misses        294      294           
  Partials       50       50           

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[ptomato]

I didn't clock this earlier but now that I think about it some more, this check should probably turn into an assertion after a table of reference years is added in tc39/proposal-intl-era-monthcode#60. Then the note will no longer be necessary.

Let's hold off on this until that issue is solved and see if this still applies.