fetch: add docs on why forbidden header names are not supported

[trivikr]

This relates to...

• Comment https://github.com/nodejs/undici/pull/3695#pullrequestreview-2350953699
• Fetch spec https://fetch.spec.whatwg.org/#concept-headers-append

Changes

Documents why undici fetch does not support forbidden header names

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• [x] I have read and agreed to the Developer's Certificate of Origin
• [ ] Tested
• [ ] Benchmarked (optional)
• [ ] Documented
• [ ] Review ready
• [ ] In review
• [ ] Merge ready