Extend automatic checkout completion with info that the checkout will…

… not be completed if any validation error occurs (saleor#1375)

* Extend automatic checkout completion with info that the checkout will not be completed if any validation error occures

* Apply code review suggestions

docs/developer/payments/transactions.mdx

@@ -412,6 +412,22 @@ Query variables:
 }
 ```
 
+:::warning
+If any issues occur during checkout validation, the checkout will not be completed.
+The checkout does not pass the validation when:
+  - contains unavailable items:
+    - one of the checkout line has missing product or variant channel listing or the price is not set;
+    - one of the product is not available for purchase;
+    - one of the product is set as not visible to the customers;
+  - billing address is not set;
+  - attached voucher is not applicable;
+  - attached gift card is not valid;
+  - shipping method is invalid;
+  - channel is inactive.
+
+You can view the specific issues using the `Checkout.problems` query. For more details, please refer to the [Checkout Problems page](/developer/checkout/problems).
+:::
+
 :::danger
 **Once this setting is enabled, you should verify if your connected services, including storefront, can handle this flow.**
 

docs/upgrade-guides/core/3-19-to-3-20.mdx

@@ -94,6 +94,14 @@ If automatic checkout completion is not enabled in a `Channel`, storefront shoul
 ```
 :::
 
+:::tip
+The checkout may not be automatically completed due to issues that arise during validation.
+
+You can identify any existing problems using the `Checkout.problems` query.
+For more information, refer to the [Checkout Problems page](/developer/checkout/problems).
+
+:::
+
 :::tip
 To determine if `Checkout` was completed you can also rely on response from `transactionInitialize` or `transactionProcess` mutations by checking the `TransactionEvent` object returned in the mutation response.