[css-values-4] Remove mention of combining number and percentage in calc(), and add clarifying issue. Fixes #2789.

Author: tabatkins

css-values-4/Overview.bs

@@ -17,7 +17,7 @@ Inline Github Issues: yes
 </pre>
 <pre class='link-defaults'>
 spec: css-backgrounds-3; type: type; text: <position>
-spec: css21; type: property; text: border-collapse
+spec: css2; type: property; text: border-collapse
 spec: css-color-4; type: value; text: currentcolor
 spec: css-cascade-4; type: at-rule; text: @import
 </pre>
@@ -748,7 +748,7 @@ Percentages: the <<percentage>> type</h3>
 	or something else.
 
 	In cases where a <<percentage>> can represent the same quantity
-	as a <a>dimension</a> or <a>number</a> in the same <a>component value</a> position,
+	as a <a>dimension</a> in the same <a>component value</a> position,
 	and can therefore be combined with them in a ''calc()'' expression,
 	the following convenience notations may be used in the property grammar:
 
@@ -764,9 +764,6 @@ Percentages: the <<percentage>> type</h3>
 	: <dfn>&lt;time-percentage></dfn>
 	:: Equivalent to <code class=prod>[ <<time>> | <<percentage>> ]</code>,
 		where the <<percentage>> will resolve to a <<time>>.
-	: <dfn>&lt;number-percentage></dfn>
-	:: Equivalent to <code class=prod>[ <<number>> | <<percentage>> ]</code>,
-		where the <<percentage>> will resolve to a <<number>>.
 
 	<div class="example">
 		For example, the 'width' property can accept a <<length>> or a <<percentage>>,
@@ -787,6 +784,10 @@ Percentages: the <<percentage>> type</h3>
 	Note: Specifications should never alternate <<percentage>> in place of a dimension
 	in a grammar unless they are <a>compatible</a>.
 
+	Note: More &lt;TYPE-percentage> productions can be added in the future as needed.
+	A &lt;number-percentage> will never be added,
+	as <<number>> and <<percentage>> can't be combined in ''calc()''.
+
 <h3 id='dimensions'>
 Numbers with Units: <a>dimensions</a></h3>