[css3-values] define 'dot' to be a device pixel

Hi,

So far as I can tell, the closest thing to a definition for "dot" in
CSS3 Values is in the caption of Figure 2:

    "Showing that more device pixels (dots) are needed to cover a 1px by
    1px area on a high-resolution device than on a low-res one"

I think it would be good if this definition were raised out of the
caption to be its own statement in normative prose somewhere. The 'dpi',
'dpcm', and 'dppx' unit definitions in §6.4 could then reference this
definition.


Thanks,
Ted