Add "closed" state.

[alvestrand]

Issue number that this PR fixes (if any). For example: 'Fixes #987654321'

What was wrong/why is this fix needed? (quick summary only)

This was missing in docs, it's listed on https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum and implemented.

Anything else that could help us review it

Example of confusion caused by this error:

https://bugs.chromium.org/p/chromium/issues/detail?id=1245777&

[github-actions]

Preview URLs

• https://pr8554.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/RTCPeerConnection/signalingState

Flaws

URL: /en-US/docs/Web/API/RTCPeerConnection/signalingState
Title: RTCPeerConnection.signalingState
on GitHub
Flaw count: 2

• macros:
  • Unknown macro domref
• broken_links:
  • Can't resolve /en-US/docs/Web/Guide/API/WebRTC

External URLs

URL: /en-US/docs/Web/API/RTCPeerConnection/signalingState
Title: RTCPeerConnection.signalingState
on GitHub

No new external URLs

[teoli2003]

Thanks!

[teoli2003]

Thanks a lot!

Looks good to me.

Congratulations upon your first merged commit here! Welcome aboard! 🎉