[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.

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−"""
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−
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−:organization: Logilab
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−:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
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−:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
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−:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
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−"""
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−distname = 'cubicweb-mycube'