[css-backgrounds] 'box-shadow' should define its 'none' value

https://drafts.csswg.org/css-backgrounds-3/#box-shadow should have
an actual definition of its 'none' value.   It currently doesn't.
(I noticed because I just added prose that would, if it had been
present, have linked to it from
https://drafts.csswg.org/css-transitions-1/#animtype-shadow-list .)

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)