[css-counter-styles-3] Extended @counter-style containing *-symbols are not invalid (#11883)

As resolved in #1682. Oversight in ba2a8cb. Fixes #9258.

Author: cdoublev

css-counter-styles-3/Overview.bs

@@ -709,8 +709,7 @@ Building from Existing Counter Styles: the ''extends'' system
 	rather than taking their initial values.
 
 	If a ''@counter-style'' uses the ''extends'' system,
-	it must not contain a '@counter-style/symbols' or '@counter-style/additive-symbols' descriptor,
-	or else the ''@counter-style'' rule is invalid.
+	it must not contain a '@counter-style/symbols' or '@counter-style/additive-symbols' descriptor.
 
 	If the specified <<counter-style-name>> is an [=ASCII case-insensitive=] match
 	for ''disc'', ''circle'', ''square'', ''disclosure-open'', or ''disclosure-closed''