docs(ngSelected): note that it doesn't interact with ngModel

Closes #14916
Related #14876

Author: Narretz

src/ng/directive/attrs.js

@@ -271,6 +271,13 @@
  * A special directive is necessary because we cannot use interpolation inside the `selected`
  * attribute. See the {@link guide/interpolation interpolation guide} for more info.
  *
+ * <div class="alert alert-warning">
+ *   **Note:** `ngSelected` does not interact with the `select` and `ngModel` directives, it only
+ *   sets the `selected` attribute on the element. If you are using `ngModel` on the select, you
+ *   should not use `ngSelected` on the options, as `ngModel` will set the select value and
+ *   selected options.
+ * </div>
+ *
  * @example
     <example>
       <file name="index.html">