[css3-images] definition of concrete object size

http://dev.w3.org/csswg/css3-images/#concrete-object-size appears to
be a definition of the term "concrete object size", but doesn't
actually define how to get a concrete object size.  (But it does
slightly confusingly link to itself.)

It should at least say that the definition of how to get a concrete
object size is in the sections below.  (Though 5.3 claims to define
it only "In the absence of more specific rules"... perhaps it could
give examples of what would provide such rules?)

-David

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