Webcomponents: Deprecation warning misleading.

[SanderElias]

In the Custom Elements Editor draft page
Are a couple of warnings prominently visible

and

While those are correct, they are incomplete and might give a visitor the idea that Custom Elements are no longer supported. (Got questions along that line!)
Perhaps it is a good idea to add some context and a URI to the current place of the spec.

Or at least mention that it's no longer active there because it's merged into HTML/DOM
cc: @domenic

[kenchris]

@annevk

[annevk]

I'm happy to land a PR that replaces that entire document with forwarding pointers.

It's a little harder for Shadow DOM I suspect as that still contains some bits that are not uplifted elsewhere. 😟