[css-transitions] Add missing statement that transition needs to be canceled.

Author: dbaron

css-transitions/Overview.html

@@ -1137,7 +1137,8 @@ <h2 id=starting><span class=secno>3. </span> Starting of transitions</h2>
         in the <a href="#after-change-style">after-change style</a>,
       </ul>
 
-     <li> Otherwise, implementations must start a new transition whose:
+     <li> Otherwise, implementations must cancel the running transition and
+      start a new transition whose:
       <ul>
        <li> <a href="#start-time">start time</a> is the time of the <a
         href="#style-change-event">style change event</a> plus the matching

css-transitions/Overview.src.html

@@ -1160,8 +1160,8 @@ <h2 id="starting">
             </ul>
           </li>
           <li>
-            Otherwise, implementations must start a new transition
-            whose:
+            Otherwise, implementations must cancel the running transition
+            and start a new transition whose:
             <ul>
               <li>
                 <span>start time</span> is