Document .direction & .relatedTarget properties of Carousel events.

Fixes #13395

docs/_includes/js/carousel.html

@@ -203,6 +203,11 @@ $('.carousel').carousel({
 
   <h3>Events</h3>
   <p>Bootstrap's carousel class exposes two events for hooking into carousel functionality.</p>
+  <p>Both events have the following additional properties:</p>
+  <ul>
+    <li><code>direction</code>: The direction in which the carousel is sliding (either <code>"left"</code> or <code>"right"</code>).</li>
+    <li><code>relatedTarget</code>: The DOM element that is being slid into place as the active item.</li>
+  </ul>
   <div class="table-responsive">
     <table class="table table-bordered table-striped">
       <thead>