[book] document __unique_together__, remove bad RQLUniqueConstraint example
RQLUniqueConstraint should be avoided whenever possible. Related to
#3753250.
#! /bin/sh -ecase "$1" in remove) if [ -x "/etc/init.d/cubicweb-ctl" ]; then invoke-rc.d cubicweb-ctl stop || true fi ;;esac#DEBHELPER#exit 0