docs(loading): add some docs on the onDismiss function

ionic/components/loading/loading.ts

@@ -36,7 +36,9 @@ import {ViewController} from '../nav/view-controller';
  * the `duration` of the loading options. By default the loading indicator
  * will show even during page changes, but this can be disabled by setting
  * `dismissOnPageChange` to `true`. To dismiss the loading indicator after
- * creation, call the `dismiss()` method on the Loading instance.
+ * creation, call the `dismiss()` method on the Loading instance. The
+ * `onDismiss` function can be called to perform an action after the loading
+ * indicator is dismissed.
  *
  * ### Limitations
  * The element is styled to appear on top of other content by setting its
@@ -71,6 +73,10 @@ import {ViewController} from '../nav/view-controller';
  *     duration: 5000
  *   });
  *
+ *   loading.onDismiss(() => {
+ *     console.log('Dismissed loading');
+ *   });
+ *
  *   this.nav.present(loading);
  * }
  *

ionic/components/loading/test/basic/index.ts

@@ -13,6 +13,10 @@ class E2EPage {
       duration: 1000
     });
 
+    loading.onDismiss(() => {
+     console.log('Dismissed loading');
+    });
+
     this.nav.present(loading);
   }