[API][DOC]Adding a tip about api's preventing methods beside GET to load non cart orders by default (#17256)

| Q               | A
|-----------------|-----
| Branch?           | 1.13
| Bug fix?           | no
| New feature?  | no
| BC breaks?      | no
| Deprecations? | no
| Related tickets | na
| License         | MIT

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Hello, 

If we add a custom API endpoint to the Orders using any method besides
GET, we end up with a very unclear 404 error.

Investigating this led us to find this "feature" which prevents us from
loading the order with a given token due to the order not being in the
state `cart`. In our case, we needed to update the customer cart with
information from an older order. So it was logical for us to use `POST`
as we are updating the cart, and it was logical to have the order token
in the URL to specify the order we need to pull the data from.

It was therefore quite frustrating to debug; I thought adding a little
tip about it in the docs wouldn't hurt any future developer coming
across this issue.

Reference to the class where this filtering is done:
https://github.com/Sylius/Sylius/blob/2.0/src/Sylius/Bundle/ApiBundle/Doctrine/ORM/QueryExtension/Shop/Order/StateBasedExtension.php

Hope I contributed as needed, with ❤️ 

Regards

docs/customization/api/adding_and_removing_endpoints.rst

@@ -23,6 +23,16 @@ And that's all, now you have a new endpoint with your custom logic.
 
     Read more about API Platform endpoint configuration `here <https://api-platform.com/docs/core/operations/>`_
 
+Good to Know
+~~~~~~~~~~~~
+
+.. tip::
+    API Platform is configured to prevent modifications to orders not in the ``cart`` state. There are only a few 
+    specific actions allowed. This is done by preventing orders not in the state ``cart`` from loading if the method 
+    is not ``GET``. So if you need to add an endpoint to an API that needs to edit orders that are not in the state 
+    ``cart``, you will need to whitelist this endpoint. This can be done by adding your API route to the 
+    ``sylius.api.doctrine_extension.order_shop_user_item.filter_cart.allowed_non_get_operations`` parameter. 
+
 How to remove an endpoint?
 --------------------------