[IMP] doc: add a tip to create a model without any table

As explained in #17386, _auto=False on a model is used for report where the
table is explicitely created in init method.
To create a model without any table, AbstractModel should be used.

Closes #19448

doc/reference/orm.rst

@@ -630,6 +630,9 @@ Model Reference
 
         If set to ``False``, override :meth:`.init` to create the database
         table
+        
+     .. tip:: To create a model without any table, inherit
+              from ``odoo.models.AbstractModel``
 
     .. attribute:: _table