[forms] propagate aguments given to .vocabulary to .choices if it's a callable. So we can properly uses vocabulary(limit=xxx) in places where we used to call .choices(limit), since field.choices is not meant to be accessed directly. Also, document expected behaviour for custom choices method for relation fields.

doc/book usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc