[FIX] doc: force_company isn't supported in master anymore.

New multi-company guidelines for 13.0 documentation explain the use of 
force_company. This is still valid in 13.0, but removed in 13.1 and 
replaced by the with_company mechanism.

closes odoo/odoo#41108

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>