Add explanation for discarded modern image outputs

[gabelivan]

Fixes #2442.

Summary

Adds a short explanation to the Modern Image Formats setting to clarify that generated AVIF/WebP images may be discarded when the generated file is larger than the original upload.

Testing

• Verified the description appears under the Image output format setting in Media Settings.
• Ran npm run lint-php.

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
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[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.27%. Comparing base (ac18b75) to head (1eedc64).
⚠️ Report is 4 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/webp-uploads/settings.php	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2518      +/-   ##
==========================================
- Coverage   69.29%   69.27%   -0.03%     
==========================================
  Files          90       90              
  Lines        7723     7726       +3     
==========================================
  Hits         5352     5352              
- Misses       2371     2374       +3     

Flag	Coverage Δ
multisite	69.27% <0.00%> (-0.03%)	⬇️
single	35.74% <0.00%> (-0.02%)	⬇️

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[westonruter]

Shouldn't this string be combined with the previous string?

[gabelivan]

I kept them separate intentionally to avoid changing the existing translatable string, since that string may already have translations. This way, only the newly added sentence needs to be translated.

That said, I’m happy to combine them if the preferred approach here is to give translators the full sentence context in a single string.