[css3-fonts] edits for animatability

Here are proposed edits for animatability lines for css3-fonts.  I
also used "see individual properties" slightly more for the
font-variant shorthand, since it's a shorthand.

It marks animatibility for font-stretch as an issue.  I tend to
think it probably should be animatable (basically just like
font-weight is), but that requires a bit more text if others agree.

(I didn't try running the preprocessor on this.)

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂