Clarify the associated CSS style sheet

Author: domfarolino

cssom-1/Overview.bs

@@ -789,6 +789,9 @@ represents a style sheet as defined by the CSS specification. In the CSSOM a
   Note: Changing the <a>media</a>'s {{MediaList/mediaText}} attribute does not
   change the corresponding attribute on the <a for=CSSStyleSheet>owner node</a>.
 
+  Note: The <a for=CSSStyleSheet>owner node</a> of a <a>CSS style sheet</a>, if non-null, is the node whose <a>associated
+  CSS style sheet</a> is the <a>CSS style sheet</a> in question.
+
 
  <dt><dfn id=concept-css-style-sheet-title>title</dfn>
  <dd>
@@ -1200,8 +1203,8 @@ To <dfn export>fetch a CSS style sheet</dfn> with parsed URL <var>parsed URL</va
 ### The {{LinkStyle}} Interface ### {#the-linkstyle-interface}
 
 The <dfn export>associated CSS style sheet</dfn> of a node is the <a>CSS style sheet</a> in the list of
-<a>document or shadow root CSS style sheets</a> of which the <a for=CSSStyleSheet>owner node</a> implements the {{LinkStyle}}
-interface.
+<a>document CSS style sheets</a> of which the <a for=CSSStyleSheet>owner node</a> is said node. This node
+must also implement the {{LinkStyle}} interface.
 
 <pre class=idl>
 interface mixin LinkStyle {