docs(ionContent): clarify content gets child scope

Closes #1392

js/angular/directive/content.js

@@ -18,6 +18,9 @@
  * directive, and infinite scrolling with the {@link ionic.directive:ionInfiniteScroll}
  * directive.
  *
+ * Be aware that this directive gets its own child scope. If you do not understand why this
+ * is important, you can read [https://docs.angularjs.org/guide/scope](https://docs.angularjs.org/guide/scope).
+ *
  * @param {string=} delegate-handle The handle used to identify this scrollView
  * with {@link ionic.service:$ionicScrollDelegate}.
  * @param {boolean=} padding Whether to add padding to the content.