Add example of normalizing N-ary operators (#532)

Author: darrnshn

css-typed-om/Overview.bs

@@ -2568,6 +2568,33 @@ CSS <<number>>, <<percentage>>, and <<dimension>> values become {{CSSNumericValu
         </pre>
     </div>
 
+    <div class=example>
+        Note that addition and multiplication are N-ary,
+        so ''calc(1px + 2px + 3px)''
+        produces the structure:
+
+        <pre>
+            CSSMathSum(
+                CSS.px(1),
+                CSS.px(2),
+                CSS.px(3)
+            )
+        </pre>
+
+        but ''calc(calc(1px + 2px) + 3px)''
+        produces the structure:
+
+        <pre>
+            CSSMathSum(
+                CSSMathSum(
+                    CSS.px(1),
+                    CSS.px(2)
+                ),
+                CSS.px(3)
+            )
+        </pre>
+    </div>
+
     Note: The value computation process may transform different units into identical ones,
     simplifying the resulting expression.
     For example, ''calc(1px + 2em)'' as a specified value