[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.
debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl