title: how to add fields in inherited notebook/page/field
body: <p>I have inherited 'sales order' from 'sales' module.</p>
  
  <p>I need to add two extra fields inside 'Order Lines' tab.</p>
  
  <p>.py</p>
  
  <pre><code>class SalesOrderInherit(models.Model):
          _inherit = "sale.order"
  type = fields.Selection([('appointment', 'Appointment'), ('walkin', 'Walk-In')], string="Type")
  </code></pre>
  
  <p>I need to add type field inside 'Order Lines' tab.</p>
  
  <p>Which table should i inherit?</p>
  
  <p>How to write  to add above field.</p>