More docs for Timer.cancel()

Notably, it's worth documenting that calling `cancel()` twice is safe.

Change-Id: I35fdf3f8a810f78f296fc96750ec9ad4d81c6fc7
Reviewed-on: https://dart-review.googlesource.com/c/83744
Commit-Queue: Jonas Jensen <jonasfj@google.com>
Auto-Submit: Jonas Jensen <jonasfj@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

sdk/lib/async/timer.dart

@@ -89,6 +89,10 @@ abstract class Timer {
 
   /**
    * Cancels the timer.
+   *
+   * Once a [Timer] has been canceled, the callback function will not be called
+   * by the timer. Calling [cancel] more than once on a [Timer] is allowed, and
+   * will have no further effect.
    */
   void cancel();