docs(ngForm): clarify usage and limitations

Author: Narretz

src/ng/directive/form.js

@@ -352,16 +352,21 @@ addSetValidityMethod({
  * @restrict EAC
  *
  * @description
- * Nestable alias of {@link ng.directive:form `form`} directive. HTML
- * does not allow nesting of form elements. It is useful to nest forms, for example if the validity of a
- * sub-group of controls needs to be determined.
- *
- * Note: the purpose of `ngForm` is to group controls,
- * but not to be a replacement for the `<form>` tag with all of its capabilities
- * (e.g. posting to the server, ...).
- *
- * @param {string=} ngForm|name Name of the form. If specified, the form controller will be published into
- *                       related scope, under this name.
+ * Helper directive that makes it possible to create control groups inside a
+ * {@link ng.directive:form `form`} directive.
+ * These "child forms" can be used, for example, to determine the validity of a sub-group of
+ * controls.
+ *
+ * <div class="alert alert-danger">
+ * **Note**: `ngForm` cannot be used as a replacement for `<form>`, because it lacks its
+ * [built-in HTML functionality](https://html.spec.whatwg.org/#the-form-element).
+ * Specifically, you cannot submit `ngForm` like a `<form>` tag. That means,
+ * you cannot send data to the server with `ngForm`, or integrate it with
+ * {@link ng.directive:ngSubmit `ngSubmit`}.
+ * </div>
+ *
+ * @param {string=} ngForm|name Name of the form. If specified, the form controller will
+ *                              be published into the related scope, under this name.
  *
  */