[repository,tests] use the new connection api (part 1/2) `add_info` should not play with the cnxset nor the mode. Static analysis showed that: * add_info is an internal api called with a non-none cnx.cnxset and also in write mode * only the altered test violated this assumption, which is now somewhat documented

[book] fix various parts of the rql section * syntax error * erroneous query * bad sphinx :func: usage

[repository] session -> cnx renaming (already done in the source object)