[selectors-4] Add parenthetical to clarify sentence.

Trying to avoid having the sentence read as
  “represents an element {capable of making sound when muted}”
instead of the intended
  “represents {an element capable of making sound} when muted”

Author: fantasai

selectors-4/Overview.bs

@@ -2483,7 +2483,7 @@ Sound State: the '':muted'' and '':volume-locked'' pseudo-classes</h3>
 
 	The <dfn>:muted</dfn> pseudo-class represents
 	an element capable of making sound
-	when that element is “muted“.
+	when that element is “muted“ (forced silent).
 	(For the <{audio}> and <{video}> elements of HTML, see [=muted=]. [[HTML]])
 
 	The <dfn>:volume-locked</dfn> pseudo-class represents