Issue with "Browser Extensions": sub-documents not clear about limited availability of API

MDN URL: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions

#### What information was incorrect, unhelpful, or incomplete?

Related issues: https://github.com/mdn/browser-compat-data/issues/8906#issue-797492653

Not incorrect exactly, but sub-documents require review in order to clarify the context of existence.

#### Specific section or headline?

All sub-documents related to "WebExtensions"

#### What did you expect to see?

When entering MDN from a search engine result, uses often access the sub-documents directly via a link on the search results page. When this occurs, the user is not clearly notified that the APIs documented here are for "WebExtensions" and will not work within the browser natively. This causes confusion unless the user happens to notice the crumb-trail mention of "Web Extensions" and actually knows what that means and how it relates to the content on the document.

It would be better if the each sub-document clearly stated near the top of the document that this API is meant for use with "Web Extensions" only.

For your consideration.

#### Did you test this? If so, how?

Search web for "windows.WindowState" navigate directly to the API sub-document. Try to determine that this API is _only_ for use with WebExtensions.

MDN Content page report details

* Folder: `en-us/mozilla/add-ons/webextensions`
* MDN URL: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions
* GitHub URL: https://github.com/mdn/content/blob/master/files/en-us/mozilla/add-ons/webextensions/index.html
* Last commit: https://github.com/mdn/content/commit/b3eebddc4c3a897597e3602655d596eed67851b6
* Document last modified: 2021-01-29T09:29:22.000Z