[FIX] base: returning a domain from an onchange

Turns out we've got an operator just for the pattern of "match value
if there's one, otherwise match everything".

Also remove an example of domain in onchange doc.

closes odoo/odoo#40938

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

odoo/addons/base/models/res_company.py

@@ -81,7 +81,10 @@ class Company(models.Model):
     street2 = fields.Char(compute='_compute_address', inverse='_inverse_street2')
     zip = fields.Char(compute='_compute_address', inverse='_inverse_zip')
     city = fields.Char(compute='_compute_address', inverse='_inverse_city')
-    state_id = fields.Many2one('res.country.state', compute='_compute_address', inverse='_inverse_state', string="Fed. State")
+    state_id = fields.Many2one(
+        'res.country.state', compute='_compute_address', inverse='_inverse_state',
+        string="Fed. State", domain="[('country_id', '=?', country_id)]"
+    )
     bank_ids = fields.One2many('res.partner.bank', 'company_id', string='Bank Accounts', help='Bank accounts related to this company')
     country_id = fields.Many2one('res.country', compute='_compute_address', inverse='_inverse_country', string="Country")
     email = fields.Char(related='partner_id.email', store=True, readonly=False)
@@ -173,13 +176,9 @@ class Company(models.Model):
 
     @api.onchange('country_id')
     def _onchange_country_id_wrapper(self):
-        res = {'domain': {'state_id': []}}
-        if self.country_id:
-            res['domain']['state_id'] = [('country_id', '=', self.country_id.id)]
         values = self.on_change_country(self.country_id.id)['value']
         for fname, value in values.items():
             setattr(self, fname, value)
-        return res
 
     @api.model
     def _name_search(self, name, args=None, operator='ilike', limit=100, name_get_uid=None):

odoo/api.py

@@ -148,7 +148,6 @@ def onchange(*args):
     .. code-block:: python
 
         return {
-            'domain': {'other_id': [('partner_id', '=', partner_id)]},
             'warning': {'title': "Warning", 'message': "What is this?", 'type': 'notification'},
         }