Fix warnings when building the trytond documentation
When building the trytond documentation there are two warnings:
trytond/doc/ref/models/fields.rst:869: WARNING: duplicate object description of trytond.model.fields.Many2Many.get_target, other instance in ref/models/fields, use :noindex: for one of them
and
trytond/doc/topics/rpc.rst:28: WARNING: Unknown directive type "TODO".