docs(ngForm): clarify the purpose of ngForm

Related to #6704 and #2513.

src/ng/directive/form.js

@@ -215,6 +215,10 @@ function FormController(element, attrs, $scope, $animate) {
  * 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.
  *