[book] document __unique_together__, remove bad RQLUniqueConstraint example
RQLUniqueConstraint should be avoided whenever possible. Related to
#3753250.
#! /bin/sh -e
#
if [ "$1" = configure ]; then
if which install-docs >/dev/null 2>&1; then
install-docs -i /usr/share/doc-base/cubicweb-doc
fi
fi
#DEBHELPER#
exit 0