[css-fonts-4] Clarify how the computed font-size is determined for size keyword

cc @emilio @rwlbuis @bkardell @bfgeek

Spec: https://drafts.csswg.org/css-fonts-4/#font-size-prop

For example Emilio gave me this example:

  <div style="font-family: serif">Serif <div style="font-family: monospace">mono</div></div>

The first div has size 16px and the second 13px in all browsers. font-size on them is the default "medium" value. Compare with:

  <div style="font-size: 16px"><div style="font-family: serif">Serif <div style="font-family: monospace">mono</div></div></div>

Currently, the spec suggests some scale factors but maybe it should make clear that the computed font-size is determined from the computed font-family, lang, user preferences etc