docs(fetch): document forbidden request headers

[nthbotast]

Summary\n- add a Forbidden headers section to the Fetch docs\n- explain why headers like connection are rejected\n- link to the Fetch spec list of forbidden request-header names\n\nFixes #1470\n\n## Validation\n- git diff --check\n- npx --yes markdownlint-cli2 docs/docs/api/Fetch.md (fails on pre-existing issues in this file: MD013 at lines 3/11/50 and MD025 for existing second H1 # Body Mixins)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.01%. Comparing base (b12081c) to head (635d77a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4871      +/-   ##
==========================================
- Coverage   93.02%   93.01%   -0.02%     
==========================================
  Files         112      112              
  Lines       35157    35157              
==========================================
- Hits        32705    32701       -4     
- Misses       2452     2456       +4     

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[KhafraDev]

If you can't bother to preview the PR before opening, why should we spend time reviewing it?