fix(instr-fetch): do not enable in Node.js; clarify in docs this instr is for web fetch only

[trentm]

A side issue from #4496 was the mistaken use of instrumentation-fetch for Node.js's fetch(). instrumentation-fetch does not intend to handle Node.js's fetch.

This change makes it so instrumentation-fetch will noop when used in Node.js.

Refs: #4496

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Theoretically I think instrumentation-fetch should be able instrument Node.js fetch, but currently it is written with dependencies on a web otel package and its tests assume web/browser. See #4333 for discussion on instrumenting Node.js' fetch().

[codecov]

Codecov Report

Merging #4498 (6d0aeb9) into main (89caef9) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4498      +/-   ##
==========================================
- Coverage   92.43%   92.40%   -0.03%     
==========================================
  Files         330      330              
  Lines        9525     9531       +6     
  Branches     2033     2036       +3     
==========================================
+ Hits         8804     8807       +3     
- Misses        721      724       +3     

Files	Coverage Δ
...s/opentelemetry-instrumentation-fetch/src/fetch.ts	95.05% <50.00%> (-1.54%)	⬇️

[weyert]

There is this PR that is relevant:
#4393

[pichlermarc]

Thanks for taking care of that. 👍