Editorial: clarify responsible browsing context

This fixes #3846, though does not clarify the situation for all callers necessarily.

Removing this concept altogether is tricky as it's used in workers for disabled scripts. #1580 tracks that.

source

@@ -79351,7 +79351,7 @@ interface <dfn>BarProp</dfn> {
      <dt>The <span>responsible browsing context</span></dt>
      <dd>
 
-      <p>Return the <span>browsing context</span> with which <var>window</var> is associated.</p>
+      <p>Return <var>window</var>'s <span data-x="window bc">browsing context</span>.</p>
 
      </dd>