feature #11134 Adding documentation of the order processor (mamazu, l…

…chrusciel)

This PR was merged into the 1.8-dev branch.

Discussion
----------

| Q               | A
| --------------- | -----
| Branch?         | 1.6
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | none
| License         | MIT

Adding documentation of the order processor visualization.

![image](https://user-images.githubusercontent.com/6213903/75117215-d26def00-566f-11ea-8de0-e779f608d2ad.png)
![image](https://user-images.githubusercontent.com/6213903/75117223-d732a300-566f-11ea-888b-f4c38a689f38.png)


Commits
-------

b202d68 Adding documentation of the order processor
221e9f4 Updated the image to make it clearer
89aebb4 Update docs/book/orders/orders.rst
ffd62ae [Docs] Fixed image display, code structure and semantics
c2e7d1c Better image
23a7d14 [Docs] Add missing tab
f676568 Better image

docs/book/orders/orders.rst

@@ -85,14 +85,23 @@ In order to change the amount of items use the **OrderItemQuantityModifier**.
 
     $this->container->get('sylius.order_item_quantity_modifier')->modify($orderItem, 3);
 
-Add the item to the order. And then call the **CompositeOrderProcessor** on the order to have everything recalculated.
+Add the item to the order. And then call the **CompositeOrderProcessor** on the order to have
+everything recalculated.
 
 .. code-block:: php
 
     $order->addItem($orderItem);
 
     $this->container->get('sylius.order_processing.order_processor')->process($order);
 
+.. note::
+
+    This **CompositeOrderProcessor** is one of the most powerful concepts. It handles whole order calculation logic and allows
+    for really granular operations over the order. It is called multiple times in the checkout process, and internally it works like this:
+
+    .. image:: ../../_images/sylius_order_processor.png
+        :align: center
+
 Finally you have to save your order using the repository.
 
 .. code-block:: php