[book] document __unique_together__, remove bad RQLUniqueConstraint example RQLUniqueConstraint should be avoided whenever possible. Related to #3753250.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl