docs: clarify UndiciHeaders validation guidance

[mcollina]

Summary\n- clarify that Undici performs protocol-level header syntax validation\n- document that callers must validate and sanitize user-provided header names and values\n- document that in the string[] header format, only indexed array elements are processed and extra array object properties are ignored\n\n## Testing\n- npm run lint

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (0a23610) to head (5957fd1).

Additional details and impacted files

@@ Coverage Diff @@ ## main #4832 +/- ## ========================================== + Coverage 93.19% 93.20% +0.01%  ========================================== Files 109 109 Lines 34222 34222 ========================================== + Hits 31893 31897 +4  + Misses 2329 2325 -4 

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[Uzlopak]

This is important, but I fear people will not notice it. I think we should put it into a warning box, if possible