[selectors] Fix the grammar for <combinator>, and add a whitespace rule for it.

Author: tabatkins

selectors-4/Overview.bs

@@ -3421,7 +3421,7 @@ Grammar</h2>
 	<dfn>&lt;simple-selector></dfn> = <<type-selector>> | <<subclass-selector>>
 
 
-	<dfn>&lt;combinator></dfn> = '>' | '+' | '~' | [ '||' ]
+	<dfn>&lt;combinator></dfn> = '>' | '+' | '~' | [ '|' '|' ]
 
 	<dfn>&lt;type-selector></dfn> = <<wq-name>> | <<ns-prefix>>? '*'
 
@@ -3463,6 +3463,7 @@ Grammar</h2>
 			of a <<pseudo-element-selector>> or a <<pseudo-class-selector>>.
 		* Between <em>any</em> of the components of a <<wq-name>>.
 		* Between the components of an <<attr-matcher>>.
+		* Between the components of a <<combinator>>.
 	* The four <a href="https://www.w3.org/TR/CSS2/selectors.html#pseudo-element-selectors">Level 2</a> <a>pseudo-elements</a>
 		(''::before'', ''::after'', ''::first-line'', and ''::first-letter'')
 		may, for legacy reasons,