[css-sizing-3] Define automatic block/inline size as terms.

Author: fantasai

css-sizing-3/Overview.bs

@@ -404,7 +404,8 @@ Sizing Values: the <<length-percentage>>, ''width/auto'' | ''max-width/none'', '
 
 		<dt><dfn for="width, height, min-width, min-height">auto</dfn>
 		<dd>
-			For 'width'/'height', specifies an <dfn export dfn for>automatic size</dfn>.
+			For 'width'/'height', specifies an <dfn export dfn for>automatic size</dfn>
+			(<dfn export dfn for>automatic [=block size=]</dfn>/<dfn export dfn for>automatic [=inline size=]</dfn>).
 			See the relevant layout module for how to calculate this.
 
 			For 'min-width'/'min-height',