[css-transitions] Add note to TransitionEvent.propertyName explaining why it's always a longhand.

This addresses the feedback in the twitter thread at:
https://twitter.com/smfr/status/839517021104545793

Author: dbaron

css-transitions/Overview.bs

@@ -1114,6 +1114,7 @@ associated with transitions.
 
 :   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-propertyName">propertyName</dfn></code> of type <code>DOMString</code>, readonly
 ::  The name of the CSS property associated with the transition.
+    <p class="note">Note:  This is always the name of a longhand property.  See 'transition-property' for how specifying shorthand properties causes transitions on longhands.</p>
 :   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-elapsedTime">elapsedTime</dfn></code> of type <code>float</code>, readonly
 ::  The amount of time the transition has been running, in seconds, when this
     event fired not including any time spent in the delay phase.  The precise