Clarify isTrusted

Per suggestion in
whatwg/html#1912 (comment).

dom.bs

@@ -835,11 +835,17 @@ true if <a>context object</a>'s <a>canceled flag</a> is set, and false otherwise
 
 <hr>
 
-The <dfn attribute for=Event><code>isTrusted</code></dfn> attribute
+<p>The <dfn attribute for=Event><code>isTrusted</code></dfn> attribute
 must return the value it was initialized to. When an
 <a>event</a> is created the attribute must be
 initialized to false.
 
+<p class="note no-backref">{{Event/isTrusted}} is a convenience that indicates whether an
+<a>event</a> is <a>dispatched</a> by the user agent (as opposed to using
+{{EventTarget/dispatchEvent()}}). The sole legacy exception is {{HTMLElement/click()}}, which causes
+the user agent to dispatch an <a>event</a> whose {{Event/isTrusted}} attribute is initialized to
+false.
+
 The <dfn attribute for=Event><code>timeStamp</code></dfn> attribute
 must return the value it was initialized to. When an
 <a>event</a> is created the attribute must be
@@ -1510,8 +1516,7 @@ initialized, and a <var>legacy target override flag</var>, run these steps:
  {{Event/cancelable}} attribute initialized to true".
 
  <p>Or, when a custom constructor is required, "<a>fire an event</a> named <code>click</code> at
- <var>target</var> using {{MouseEvent}} with its {{Event/isTrusted}} attribute initialized to
- false".
+ <var>target</var> using {{MouseEvent}} with its {{UIEvent/detail}} attribute initialized to 1".
 
  <p>Ocassionally the return value is important: