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

[KhafraDev]

I find the comments harder to read considering it merges two different spec functions (I understand they're out of date). I don't see any value in documenting why we don't implement forbidden headers either as it's well-known and documented already.

[trivikr]

The benefit is for developers who read the code on while going through the implementation.

[Uzlopak]

Why do we deviate from the spec?

[trivikr]

The source code is for https://fetch.spec.whatwg.org/#concept-headers-append The link is provided in "relates to" section, as well as existing documentation, i.e. line 83, in the source code.

The screenshot provided is for https://fetch.spec.whatwg.org/#headers-class