Clarify support status of CPU image access

[tangobravo]

I think there were some issues with CPU image access early on in ARCore on some very specific devices that have now been resolved.

The native reference docs still have a mention of this though:
https://developers.google.com/ar/reference/c/group/ar-frame#arframe_acquirecameraimage

Returns the CPU image for the current frame.

Caller is responsible for later releasing the image with ArImage_release. Not supported on all devices (see https://developers.google.com/ar/devices).

The supported device page has no mention of "CPU", so is it OK to assume this is now supported everywhere and I don't have to worry about a GPU read back fallback path?

I note the Java docs don't mention anything about unsupported devices any more.