[css-mixins-1] Consistently return a syntax string (#12245)

Perhaps these functions should indeed return a serialized <css-type>
(#11908), but until then they should always return a valid syntax string.

Co-authored-by: Anders Hartvoll Ruud <[email protected]>

Author: andruud

css-mixins-1/Overview.bs

@@ -822,7 +822,7 @@ interface CSSFunctionRule : CSSGroupingRule {
 		The [=custom function/return type=] of the [=custom function=],
 		represented as a [[css-properties-values-api-1#syntax-strings|syntax string]].
 		If the [=custom function=] has no return type,
-		returns <code>"type(*)"</code>.
+		returns <code>"*"</code>.
 </dl>
 
 
@@ -843,7 +843,7 @@ dictionary FunctionParameter {
 	<dd>
 		The [=parameter type|type=] of the [=function parameter=],
 		represented as a [[css-properties-values-api-1#syntax-strings|syntax string]],
-		or <code>"type(*)"</code> if the [=function parameter|parameter=] has no type.
+		or <code>"*"</code> if the [=function parameter|parameter=] has no type.
 
 	<dt>defaultValue
 	<dd>