[css-mixins-1] Allow omitting custom function arguments (w3c#11816)

Author: cdoublev

css-mixins-1/Overview.bs

@@ -388,7 +388,7 @@ whose function name starts with two dashes (U+002D HYPHEN-MINUS).
 Its syntax is:
 
 <pre class="prod informative" nohighlight>
-	&lt;dashed-function> = --*( <<declaration-value>># )
+	&lt;dashed-function> = --*( <<declaration-value>>#? )
 </pre>
 
 A <<dashed-function>> can only be used where ''var()'' is allowed.