[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.
#!/bin/sh
rm -f web/test/tmpdb*
rm -f web/tali18n.py
rm -f applications/*/test/tmpdb*
rm -f applications/*/tali18n.py
rm -f applications/*/i18n/*_full.po
rm -f applications/*/data/Schema.dot