[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.
Additional services
===================
In this chapter, we introduce services crossing the *web -
repository - administration* organisation of the first parts of the
CubicWeb book. Those services can be either proper services (like the
undo functionality) or mere *topical cross-sections* across CubicWeb.
.. toctree::
:maxdepth: 2
undo