Add note that WebRTC does not work offline on file: protocol

### MDN URL

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

### What specific section or headline is this issue about?

_No response_

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

No information about using WebRTC offline.

No information about using WebRTC offline and on `file:` protocol.

### What did you expect to see?

WebRTC does not work offline on `file:` protocol.

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

https://bugzilla.mozilla.org/show_bug.cgi?id=831926

### Do you have anything more you want to share?

_No response_

### MDN metadata

Page report details

* Folder: `en-us/web/api/webrtc_api`
* MDN URL: https://developer.mozilla.org/en-US/docs/Web/API/WebRTC_API
* GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/api/webrtc_api/index.md
* Last commit: https://github.com/mdn/content/commit/19dbdc2139ab1c68651da2b9338061d64567665d
* Document last modified: 2022-10-30T06:16:29.000Z