[css-mixins-1] Explain comma-containing values as single argument (#11451)

Author: andruud

css-mixins-1/Overview.bs

@@ -225,8 +225,6 @@ Its syntax is:
 	--*( <<declaration-value>># )
 </pre>
 
-Issue: Mention semicolon upgrades.
-
 A <<dashed-function>> can only be used where ''var()'' is allowed.
 
 If a property contains one or more <<dashed-function>>s,
@@ -334,7 +332,21 @@ Evaluating Custom Functions {#evaluating-custom-functions}
 			::  The |parameter|'s [=default value=].
 		</dl>
 	3. Return |value|.
+</div>
 
+<div class='example'>
+	A [=comma-containing productions|comma-containing value=]
+	may be passed as a single argument
+	by wrapping the value in curly braces, <code>{}</code>:
+
+	<pre class='lang-css'>
+	@function --max-plus-x(--list, --x) {
+	  result: calc(max(var(--list)) + var(--x));
+	}
+	div {
+	  width: --max-plus-x({ 1px, 7px, 2px }, 3px); /* 10px */
+	}
+	</pre>
 </div>
 
 <div algorithm>