Editorial: Add guidelines for external specs about CORS, cross-origin isolation and TAO

[noamr]

Add a section to "using fetch elsewhere" that describes in a nutshell what new standards that invoke fetch should take
into the consideration when integrating with the same-origin policy protections enforced in the Fetch standard.

• At least two implementers are interested (and none opposed):
  • Prose only, but would be good to have some cross-implementor eyes on it.
• Tests are written and can be reviewed and commented upon at:
  • N/A: prose only
• Implementation bugs are filed:
  • N/A: prose only
• MDN issue is filed: …
  • N/A: prose only
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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[domfarolino]

Looks great, thanks so much!

[jyasskin]

Thanks for writing this!

[noamr]

@annevk care to take a look?