[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/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared