[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.

""" CW - nevow/twisted client

:organization: Logilab
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:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""