[css-device-adapt] Definition/explanation of device-width/device-height

[patrickhlauke]

While device-width/device-height are referenced in terms of what they translate to, the spec may benefit from a small (non-normative?) note that explains what these values actually do in the old meta viewport model - trigger the "ideal viewport" (as PPK called it https://www.quirksmode.org/mobile/metaviewport/ ...a term that doesn't seem to be canonically defined anywhere, not even in https://github.com/WICG/ViewportAPI), i.e. the viewport / mapping of CSS pixels to hardware pixels determined by the device/OS/UA to be the "best" mapping / usually the mapping closest to the reference pixel https://www.w3.org/TR/css3-values/#reference-pixel based on the device's general use and viewing distance