Move fetchpriority references to HTML and fetch specs

[pmeenan]

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/fetchPriority

What specific section or headline is this issue about?

fetchpriority attribute

What information was incorrect, unhelpful, or incomplete?

The priority-hints spec has been obsoleted and the IDL changes have been added directly to the HTML and fetch specs.

What did you expect to see?

• The fetchpriority attribute on the img, link and script elements has been merged into the main HTML spec.
• The fetchpriority attribute has been removed from the iframe element.
• The fetchpriority attribute of fetch's RequestInit has been moved to the fetch spec.
• The attribute is no longer experimental and is now part of the main specs.

Do you have any supporting links, references, or citations?

No response

Do you have anything more you want to share?

I'll be submitting PR's for the content and browser-compat-data repos, just wanted to have an issue to reference.