doc: document Node.js fetch() differences from standard

[Aanushka001]

This PR adds documentation for intentional differences between Node.js's
fetch() implementation (based on undici) and the WHATWG Fetch Standard,
covering:

• new Response(asyncIterable) as an addition to the standard
• Cookies handling differences
• No forbidden headers enforcement
• No CORS checks

Refs: #52163

[bakkot]

There are at least two open PRs for this already: #61837 #62236

[Aanushka001]

Hi @Aanushka001! 👋 Thank you for taking the time to contribute documentation for the Node.js fetch() differences — it's great to see interest in this area!

I wanted to let you know that this PR overlaps significantly with two existing pull requests that have already been submitted:

1. PR doc: document fetch() differences from the Fetch Standard #62236 by @kovan (open) — This is the most comprehensive PR, documenting 6 differences including:

   • No CORS enforcement
   • Fewer forbidden headers (with Host still restricted — corrected per @bakkot's review)
   • Response accepts async iterables
   • Response bodies must be consumed to avoid connection leaks
   • Content-Encoding layer limit of 5
   • Manual redirect returns actual response

2. PR doc: add section about fetch() differences from standard #61837 by @akachi10 (open) — Covers async iterable response bodies, cookies handling, and forbidden headers restriction removal.

All 4 items covered in your PR (new Response(asyncIterable), cookies handling, no forbidden headers enforcement, no CORS checks) are already addressed by these earlier PRs — particularly #62236 which is more comprehensive.

Also, a small note: your PR mentions "no forbidden headers enforcement," but this was corrected in #62236 by @bakkot's review — it should be "fewer forbidden headers, with Host still restricted," as Node.js does still restrict certain headers like Host.

Given this overlap, would you consider closing this PR in favor of the earlier, more comprehensive #62236 (and partially #61837)? Your help reviewing or contributing to those existing PRs would be very valuable! Thank you again for your work on this. 🙏

[Aanushka001]

Closing this - it's a duplicate of #62236 which already covers these differences more comprehensively.