document collection.reset() with no args.

index.html

@@ -1367,6 +1367,11 @@ Accounts.fetch();
 </script>
 </pre>
 
+    <p>
+      Calling <tt>collection.reset()</tt> without passing any models as arguments
+      will empty the entire collection.
+    </p>
+
     <p id="Collection-create">
       <b class="header">create</b><code>collection.create(attributes, [options])</code>
       <br />