[connection] provide some missing documentation bits The previous changesets remove information about session data and transaction data. We provide the necessary entries in the docstring of the Connection class. Related to #3933480.

[repoapi,session] remove all session-as-cnx backward compat The `dbapi` being gone, we now can drop the session object bw-compatibility layer. This will allow further simplifications, such as folding ClientConnection and Connection (without too much pain), and then having persistent sessions. Related to #3933480.