[css-size-adjust] Specifying when text-size-adjust applies

text-size-adjustment currently only applies on "small displays" which is
ambiguous. I'd like to change "small display" to be two requirements:
1) The device has pan and pinch-zoom functionality such as a touchscreen.
2) The 'actual viewport' is larger than the 'initial viewport' as defined
by https://www.w3.org/TR/css-device-adapt-1/#initial-viewport.
Without these requirements, no adjustment would occur even when
text-size-adjust is specified.

One of the problems text-size-adjustment solves is having to pan to read
long lines of text designed for larger displays, and these new requirements
more closely match that.