[FIX] stock_account: fix rounding adjustment

This fix (odoo/odoo#46850) was too permissive and fix too much
stock valuation issues (other than rounding error) under the hood.
The problem of the "too permissive" is will hide some errors
without get any feedback about them. Also, it changes the SVL value
without any explanation.

To avoid the first issue, apply a threshold on the rounding adjustment.
This threshold is '(quantity_out * smallest_value_of_the_currency) / 2'
(`smallest_value_of_the_currency` = `rounding` field of the related
currency).
Also to improve debugging, add a line in the description
about the rounding adjustment done in a SVL

task-2452786

closes odoo/odoo#65567

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>