[ADD] accounting: How to modify a validated invoice?

https://docs.google.com/document/d/19IeaWHBKbBSJUaGnoMdP4io-Qm1DclNGgsLr_UCU9Ok/edit#heading=h.456xgexhcza7

accounting/receivables/customer_invoices.rst

@@ -6,9 +6,9 @@ Customer Invoices
    :titlesonly:
 
    customer_invoices/overview
-   customer_invoices/customize
    customer_invoices/cash_discounts
    customer_invoices/payment_terms
    customer_invoices/installment_plans
    customer_invoices/deferred_revenues
    customer_invoices/refund
+   customer_invoices/modify_validated

accounting/receivables/customer_invoices/customize.rst

@@ -1,3 +0,0 @@
-===========================================
-How to customize header/footer of invoices?
-===========================================

accounting/receivables/customer_invoices/modify_validated.rst

@@ -0,0 +1,55 @@
+==================================
+How to modify a validated invoice?
+==================================
+
+In most accounting legislation over the word, it's not allowed to modify
+an invoice that is validated and sent to the customer. However, you
+sometimes need to change an invoice for different reasons: the sale has
+changed, the made a mistake while recording the invoice, etc.
+
+Thus, the right way to modify an invoice is to:
+
+1. Refund the original invoice
+2. Create a new invoice
+
+Odoo's refund mechanism helps you handle the whole process in just a few
+clicks.
+
+Modify an invoice
+=================
+
+If your invoice is still in draft, you can modify it the way you want.
+However, if your invoice is validated, you can not change it anymore. If
+you need to modify it, the right operations to do are:
+
+1. Refund the original invoice;
+2. Reconcile the original invoice with the refund to void them;
+3. Create a new draft invoice, a copy of the original;
+4. Validate the new invoice.
+
+All those steps are automated by Odoo. All you have to do is to click on
+the Refund button on an invoice.
+
+.. image:: ./media/modify01.png
+  :align: center
+
+In the refund dialog, select the option "Modify: create a refund,
+reconcile and create a draft invoice". Once you click on the "Create
+Refund" button, Odoo will void your existing invoice and create a new
+draft invoice that is a duplicate of the original one.
+
+Edit this new draft invoice and validate it when it's correct.
+
+.. note::
+
+	if you already sent the original invoice to your customer, you should send
+	the new invoice and the refund to the customer so that he gets all the documents.
+
+.. seealso::
+	
+	* :doc:`refund`
+
+.. rst-class:: text-muted
+
+| Written by Fabien Pinckaers (Odoo)
+| Proofread by Aline Preillon (Odoo)