[FIX] models: docstring typo.

closes #4163
[FIX] fields: make sure fields are set up before using them

Non-setup fields could cause problems in two places:
- when traversing the chain of fields in related fields;
- when adding recomputation triggers on inverse fields

Both issues are fixed by this patch.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64