[css-values-5][editorial] Add cross-reference to var() notation.

Author: fantasai

css-values-5/Overview.bs

@@ -1369,6 +1369,13 @@ Toggling Between Values: ''toggle()''</h3>
 		which may not be what was intended.
 	</div>
 
+<h3 id="var-notation">
+Custom Property References: the ''var()'' notation</h3>
+
+	The ''var()'' notation substitutes the value of a [=custom property=],
+	see the <a href="https://www.w3.org/TR/css-variables/#using-variables">CSS Custom Properties for Cascading Variables Module</a>.
+	[[CSS-VARIABLES]]
+
 <!-- Big Text: attr()
 
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