[IMP] purchase: usability improvements

- Task: https://www.odoo.com/web#id=5342&view_type=form&model=project.task&action=333&active_id=131
great. You know that sort of things may seems unimportant details. But when developers in the US or other English speaking countries browse the code, it really makes a difference to have a good English. As these hackers are the opinion leaders, if the code looks ridiculous to them, there is no way it get a good reputation. So glad to see this improving and kudos for the work.