docs: clarify fetch and FormData pairing

[mcollina]

Summary

• document that fetch and FormData should come from the same implementation
• add examples for built-in globals, undici imports, and install() globals
• clarify that install() installs the matching undici globals together

Testing

• not run (docs only)

Closes #4918

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.90%. Comparing base (9766c6d) to head (d9f2b61).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4922   +/-   ##
=======================================
  Coverage   92.90%   92.90%           
=======================================
  Files         112      112           
  Lines       35667    35689   +22     
=======================================
+ Hits        33136    33158   +22     
  Misses       2531     2531           

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