[IMP] stock: Improve tooltip for `Alert date` field on production lot

PURPOSE
=======

confusing tooltip on "Alert Date".

SPECIFICATION
=============

Replace the tooltip on alert date by : "Date used to determined what are the expired lots and serial numbers using the filter "Expiration Alerts".

addons/product_expiry/models/production_lot.py

@@ -15,7 +15,7 @@ class StockProductionLot(models.Model):
     removal_date = fields.Datetime(string='Removal Date',
         help='This is the date on which the goods with this Serial Number should be removed from the stock.')
     alert_date = fields.Datetime(string='Alert Date',
-        help="This is the date on which an alert should be notified about the goods with this Serial Number.")
+        help='Date to determine the expired lots and serial numbers using the filter "Expiration Alerts".')
     product_expiry_alert = fields.Boolean(compute='_compute_product_expiry_alert', help="The Alert Date has been reached.")
 
     @api.depends('alert_date')