Merge: modelize: `missing-doc` on attributes

missing-doc only fires on public properties.
But attributes are always Private, so consider the getter method instead.

It make sense since the documentation on an attribute documents more the
getter than the attribute.

Pull-Request: #800
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/modelize/modelize_property.nit
Simple merge