Document CameraFeed platform limitations

[tetrapod00]

Fixes godotengine/godot-docs#9530.

CameraServer says:

[b]Note:[/b] This class is currently only implemented on Linux, macOS, and iOS, on other platforms no [CameraFeed]s will be available. To get a [CameraFeed] on iOS, the camera plugin from [url=https://github.com/godotengine/godot-ios-plugins]godot-ios-plugins[/url] is required.

Rather than copy the whole note, I listed the supported platforms and linked to CameraServer to fully document the limitations. Alternately I could copy the note entirely; not sure which is better to be translation and maintenance friendly.

[Mickeon]

You may want to copy the entire note because the iOS case is pretty... notable.

not sure which is better to be translation and maintenance friendly.

It is unfortunate that we don't have snippets of documentation that are easier to maintain, but it is what it is for now.

[tetrapod00]

I duplicated the note exactly. Since I was already doing so, I made a minor change to the string in both places.

[akien-mga]

Thanks!