[FIX] l10n_latam_invoice_document: document_type default value

The detected errors is about a normal field
(l10n_latam_document_type_id) which is written by a computed method
(_compute_l10n_latam_documents) making that the normal field value be
overwritten the value given by the user everytime the compute field is
re computed with the default value given by the compute.

We remove this write n the compute method and now we use and onchange
instead.

IMPORTANT: the l10n_latam_available_document_type_ids computed field is
changed from a m2m to a o2m to be able to use it on the new onchange

closes odoo/odoo#38973

X-original-commit: 96c70bb1
Signed-off-by: Josse Colpaert <jco@openerp.com>