Document which `Units` variants are valid for each property

[nicoburns]

I believe that not all variants of Units are valid for every style property which uses the Units type. It would be very helpful to have documentation stating which combinations and valid/invalid.

[geom3trik]

Agreed. I wonder if maybe new types should be used for those properties instead? The ones that come to mind are min/max constraints (where auto doesn't do anything), and border_ properties (where only pixels/percentage is used). In fact in vizia border is expressed with a LengthOrPercentage type which is then converted to Units during layout.