[css-color-4] Improve description of Highlight/HighlightText

Author: fantasai

css-color-4/Overview.bs

@@ -928,9 +928,9 @@ System Colors</h3>
 		<dd><span class="swatch" style="--color: fieldtext"></span>&nbsp;Text in input fields.
 
 		<dt><dfn>Highlight</dfn>
-		<dd><span class="swatch" style="--color: highlight"></span>&nbsp;Background of item(s) selected in a control.
+		<dd><span class="swatch" style="--color: highlight"></span>&nbsp;Background of selected items/text.
 		<dt><dfn>HighlightText</dfn>
-		<dd><span class="swatch" style="--color: highlighttext"></span>&nbsp;Text of item(s) selected in a control.
+		<dd><span class="swatch" style="--color: highlighttext"></span>&nbsp;Text of selected items/text.
 
 		<dt><dfn>Mark</dfn>
 		<dd><span class="swatch" style="--color: mark"></span>&nbsp;Background of text that has been specially marked