[ADD] accounting: provide information on the tax carryover feature

closes odoo/documentation#2889

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

content/applications/finance/accounting/reporting/overview.rst

@@ -10,3 +10,4 @@ Overview
    overview/main_reports
    overview/customize
    overview/data_inalterability
+   overview/tax_carryover

content/applications/finance/accounting/reporting/overview/tax_carryover.rst

@@ -0,0 +1,38 @@
+=============
+Tax carryover
+=============
+
+When performing tax reports, the **tax carryover** feature allows carrying amounts from one period
+to another without creating new entries.
+
+It has been created to meet the legal requirements of specific locations, where amounts must be
+transferred from period to period (for example, because the total of the line is negative).
+
+The feature is activated by default in countries where it is required, such as Belgium, France, and
+Italy. There is no specific configuration required.
+
+Let’s take an example of a Belgian company that created a credit note of 100 for one of their
+customers. The due tax is 21%.
+
+.. image:: tax_carryover/belgian-example.png
+   :align: center
+   :alt: Illustration with a credit note
+
+In this case, as per local regulation, grid 81 of the tax report may contain a negative amount. But
+it must be declared to the government as zero, and the negative amount should be carried over to the
+next period.
+
+If we go to :menuselection:`Accounting app --> Reporting --> Tax Report`, a pop-up on line 81
+explains that the amount will be carried over in the next period.
+
+.. image:: tax_carryover/pop-up.png
+   :align: center
+   :alt: pop-up message stating the amount will be carried over to the next period
+
+At the time of the tax closing period, the tax report shows that the amount was carried over from
+the previous period. It also indicates the amount that will be carried over to this line in the next
+period based on the existing transactions and the carryover from the previous period.
+
+.. image:: tax_carryover/tax-return.png
+   :align: center
+   :alt: Illustration of the tax return