[IMP] orm : add info about boolean default

[asno-odoo]

When a default value for boolean is set as False, the values of new column for existing records is set to NULL. In python, FALSE and NULL might mean same thing in certain contexts but its not the case for Sql(we allow to add sql constraints).

concerned code: https://github.com/odoo/odoo/blob/15.0/odoo/models.py#L2767

[robodoo]

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