[selectors] Add back requirement that ID selector be an ident.

Author: tabatkins

selectors/Overview.bs

@@ -3133,6 +3133,7 @@ Grammar</h2>
 		can, for legacy reasons,
 		be written using the <<pseudo-class-selector>> grammar,
 		with only a single ":" character at their start.
+	* In <<id-selector>>, the <<hash-token>>’s value must be an <a>identifier</a>.
 
 	Issue: Should I consider the "only one pseudo-element per complex selector, at the end of the last compound selector" rule now trashed?
 	In practice, all implementations violate that rule to at least some extent.