[connection] eliminate ClientConnection leftovers
* in the documentation
* in the docstrings
* in code notations (e.g. clt_cnx -> cnx)
Related to #3837233.
From now, there should be just one ClientConnection left in repoapi.py.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/