[selectors] Clarify that whitespace is required if you omit the combinator. w3c#7027

Author: tabatkins

selectors-4/Overview.bs

@@ -3498,6 +3498,11 @@ Grammar</h2>
 		* Between <em>any</em> of the components of a <<wq-name>>.
 		* Between the components of an <<attr-matcher>>.
 		* Between the components of a <<combinator>>.
+
+		Whitespace is <em>required</em>
+		between two <<compound-selector>>s
+		if the <<combinator>> between them is omitted.
+		(This indicates the descendant combinator is being used.)
 	<li id="single-colon-pseudos">
 		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'')
@@ -3513,13 +3518,6 @@ Grammar</h2>
 	for the selector to be considered valid.
 	See [[#invalid]] for additional rules for parsing selectors.
 
-	Note: The grammar above states that a combinator is optional
-	between two <<compound-selector>>s in a <<complex-selector>>.
-	This is only for grammatical purposes,
-	as the <a>CSS Value Definition Syntax</a>'s lax treatment of whitespace
-	makes it difficult to indicate that a grammar term can <em>be</em> whitespace.
-	"Omitting" a combinator is actually just specifying the <a>descendant combinator</a>.
-
 	Note: In general,
 	a <<pseudo-element-selector>> is only valid
 	if placed at the end of the last <<compound-selector>>