[selectors] Put the column combinator back into the grammar. #9544

Author: tabatkins

selectors-4/Overview.bs

@@ -3601,7 +3601,7 @@ Grammar</h2>
 	<dfn>&lt;simple-selector></dfn> = <<type-selector>> | <<subclass-selector>>
 
 
-	<dfn>&lt;combinator></dfn> = '>' | '+' | '~'
+	<dfn>&lt;combinator></dfn> = '>' | '+' | '~' | [ '|' '|' ]
 
 	<dfn noexport>&lt;wq-name></dfn> = <<ns-prefix>>? <<ident-token>>
 	<dfn noexport>&lt;ns-prefix></dfn> = [ <<ident-token>> | '*' ]? '|'