[css-values] Generify the handling of units in the attr() type arg.

Author: tabatkins

css-values/Overview.bs

@@ -1921,27 +1921,8 @@ Ian's proposal:
 			and the attribute is out of range (e.g. a negative length or 180deg).
 			If the unit is a relative length, it must be computed to an absolute length.
 
-		<dt>''em''
-		<dt>''ex''
-		<dt>''px''
-		<dt>''rem''
-		<dt>''vw''
-		<dt>''vh''
-		<dt>''vmin''
-		<dt>''vmax''
-		<dt>''mm''
-		<dt>''cm''
-		<dt>''in''
-		<dt>''pt''
-		<dt>''pc''
-		<dt>''deg''
-		<dt>''grad''
-		<dt>''rad''
-		<dt>''ms''
-		<dt>''s''
-		<dt>''Hz''
-		<dt>''kHz''
 		<dt>''%''
+		<dt>A keyword matching one of the <<length>>, <<angle>>, <<time>>, or <<frequency>> units
 		<dd>
 			The attribute value must parse as a NUMBER CSS token,
 			and is interpreted as a <a>dimension</a> with the specified unit.