[ADD] accounting: Add page 'Reverse an accounting entry automatically'

Original document: https://docs.google.com/document/d/1luJ5xVAM0BIgKI_unpV1mPQrE5p9aJC3I1ZiqngLpeU/edit

accounting/others/adviser.rst

@@ -8,3 +8,4 @@ Adviser
    adviser/assets
    adviser/fiscalyear
    adviser/budget
+   adviser/reverse_entry

accounting/others/adviser/reverse_entry.rst

@@ -0,0 +1,20 @@
+=========================================
+Reverse an accounting entry automatically
+=========================================
+
+You may want to automatically reverse an accounting entry at a specific
+date.
+
+This is, for example, used when you make a provision (e.g. provision for
+bad debt). When making the accounting entry for the provision, the
+amount you entered is only an estimated amount. You will only be sure of
+the amount at the end of the fiscal year. That's why, at that point, you
+want the accounting entry to be reversed to be able to enter the real
+loss that occurred.
+
+For this, you just have to tick the box "Reverse Automatically" on the
+journal entry and to define a reversal date. The accounting entry will
+be reversed at that date.
+
+.. image:: media/reverse_entry01.png
+    :align: center