docs: add restricted header info to ClientRequest docs

[VerteDinde]

Description of Change

Addresses #21148. (cc @nornagon)

Beginning in Electron 7, trying to set a Host header when creating a request with net throws a net::ERR_INVALID_ARGUMENT error. This is due to a Chromium change introduced in 7, where certain header types are now restricted from being set by a consumer.

It doesn't seem like we'll be able to resolve this without patching Chrome directly, but this PR updates our docs to give developers a warning about what headers are restricted/will throw an error if set, and where they can find more information.

Note: I didn't add any release notes since this is a small doc change for something that has been present since 7.0.0, but happy to add a note if it would be helpful!

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.
• This is NOT A BREAKING CHANGE. Breaking changes may not be merged to master until 11-x-y is branched.

Release Notes

Notes: none

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[release-clerk]

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