docs: clarify fetch class compatibility

[vibhor-aggr]

Fixes #3071.

This documents the supported pattern for fetch-related classes when Node.js' built-in Undici implementation and an installed undici package are both present.

The change clarifies that fetch, Request, Response, Headers, and FormData values should come from the same implementation:

• all Node.js built-in globals,
• all direct imports from the same installed undici package, or
• globals replaced together by undici.install().

It also calls out the specific failure mode from the issue: a Request created by Node.js' built-in global Request is not guaranteed to work with fetch imported from a different undici package version, and the reverse is unsupported too.

Validation:

• git diff --check
• npm run lint
• npm run test:typescript

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (ecf714f) to head (4aacff7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5549   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files         110      110           
  Lines       37418    37418           
=======================================
  Hits        34967    34967           
  Misses       2451     2451           

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