[FIX] product: help for field standard_price

Step to reproduce:
1) create a product with, the "Product Type" as
"Storable Product" + A "Product Category" with a "Costing Method"
set in "FIFO" mode and the Inventory Valuation in "Automated".

2) create a Purchase Order for 1 unit of that product and set the unit
price to 10.00. Confirm Order -> Receive Products -> Validate
(the delivery) -> Apply

3) create a second Purchase Order for 1 unit of the same product but
set the unit price to a different price, like 20.00. Confirm Order ->
Receive Products -> Validate (the delivery) -> Apply

4) create a Sales Orders for 1 unit -> Confirm -> Smart button
Delivery -> Validate -> Apply

5) check the cost on the product form of that product.

On the V13 it will be $10, so its the last unit that left the stock
On V14 and upper, it will be $20, so the next product that will leave
the stock.

The help popup for the field "standard_price" doesn't reflect the
correct behavior. It's the standard price for the next unit instead
of the last one.

closes odoo/odoo#82678

Signed-off-by: William Henrotin (whe) <whe@odoo.com>