[book] document __unique_together__, remove bad RQLUniqueConstraint example
RQLUniqueConstraint should be avoided whenever possible. Related to
#3753250.
/**
* functions specific to cubicweb on google appengine
*
* :organization: Logilab
* :copyright: 2008-2010 LOGILAB S.A. (Paris, FRANCE), all rights reserved.
* :contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
*/
/**
* .. function:: rql_for_eid(eid)
*
* overrides rql_for_eid function from htmlhelpers.hs
*/
function rql_for_eid(eid) {
return 'Any X WHERE X eid "' + eid + '"';
}