[IMP] orm: add warning about model extension

When a model is defined with `_inherit` using a list, Odoo will do a check with the class name to set the `_name` of the model being extended. This is different than the previous versions and can be misleading

Part-of: odoo/documentation#14979
Signed-off-by: Emanuele Maruzzi (emi) <emi@odoo.com>

content/developer/reference/backend/orm.rst

@@ -1249,6 +1249,9 @@ will yield::
 
     {'name': "A", 'description': "Extended"}
 
+.. warning:: When :attr:`~odoo.models.Model._inherit` is set to a string,
+    then :attr:`~odoo.models.Model._name` is set to the same value,
+    unless `_name` is explicitly set.
 
 .. note::