[book][forms] Adding details more on how data gets to the objects and how to persist in Doctrine

This was per comments from @dot-i-fy

Author: weaverryan

book/forms.rst

@@ -219,6 +219,23 @@ Now, when submitting the form, the controller binds the submitted data to the
 form, which translates that data back to the ``name`` and ``price`` properties
 of the ``$product`` object. This all happens via the ``bindRequest()`` method.
 
+.. note::
+
+ As soon as ``bindRequest()`` is called, the submitted data is transferred
+ to the underlying object immediately. For example, imagine that ``Foo``
+ is submitted for the ``name`` field:
+
+ .. code-block:: php
+
+ $product = new Product();
+ $product->name = 'Test product';
+ 
+ $product->bindRequest($this->get('request'));
+ echo $product->name;
+
+ The above statement will echo ``Foo``, because ``bindRequest`` ultimately
+ moves the submitted data back to the ``$product`` object.
+
 This controller follows a common pattern for handling forms, and has three
 possible paths:
 
@@ -648,14 +665,20 @@ when the form is valid:
 .. code-block:: php
 
  if ($form->isValid()) {
- // persist the $product object to the database
- // or do anything else you need to do
+ $em = $this->get('doctrine')->getEntityManager();
+ $em->persist($product);
+ $em->flush();
 
  return $this->redirect($this->generateUrl('store_product_success'));
  }
 
 For more information, see the :doc:`Doctrine ORM chapter</book/doctrine/orm>`.
 
+The key thing to understand is that when the form is bound, the submitted
+data is transferred to the underlying object immediately. If you want to
+persist that data, you simply need to persist the object itself (which already
+contains the submitted data).
+
 If the underlying object of a form (e.g. ``Product``) happens to be mapped
 with the Doctrine ORM, the form framework will use that information - along
 with the validation metadata - to guess the type of a particular field.