[css-nesting-1] Clarify what exactly is preserved. Also rephrase note for clarity.

Author: tabatkins

css-nesting-1/Overview.bs

@@ -577,16 +577,20 @@ Syntax {#syntax}
 		a <<delim-token>> with the value "&" (U+0026 AMPERSAND) was encountered.
 
 		Note: This is phrased in this explicit manner
-		so as to catch cases like '':is(&:unknown, .bar)'',
-		which discards its first argument during parsing
-		(since it contains an unknown pseudo-class,
-		and thus is an [=invalid selector=]).
-		A selector like this still [=contains the nesting selector=].
+		so as to catch cases like '':is(:unknown(&), .bar)'',
+		where an unknown selector
+		(which, being unknown, we have no way of knowing
+		whether the argument is <em>meant</em> to be parsed as a selector or not)
+		is the only part of the selector that contains an ''&''.
+		As that <em>might</em> be a perfectly valid selector
+		that's only supported by newer browsers,
+		and we don't want parsing to be dependent on unrelated versioning issues,
+		we treat it as still [=containing the nesting selector=].
 	</div>
 
 	If a <<forgiving-selector-list>> has an item that [=contains the nesting selector=]
 	but is invalid,
-	it is preserved exactly as-is
+	that item is preserved exactly as-is
 	rather than being discarded.
 	(This does not change the matching behavior of the selector--
 	an invalid selector still fails to match anything--