Add Picture in Picture window to note on local playback devices

[chrisn]

For context, see https://github.com/w3c/picture-in-picture/issues/191.

The Picture in Picture spec currently says:

The [Remote-Playback] specification defines a local playback device and a local playback state. For the purpose of Picture-in-Picture, the playback is local and regardless of whether it is played in page or in Picture-in-Picture.

Media WG would like to move this language to the Remote Playback API to avoid having a patch in the Picture in Picture spec.

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