effectively no documentation for get_children / the visitors module

[ben]

... even though traversing parts of a query is necessary for getting horizontal sharding right (and is even exhibited in one of the examples).

[Mike Bayer]

there's only a bit of API level documentation and it seems like sql/visitors is currently not there. it's not for the novice user by any means :). horizontal sharding API was always kind of an experiment in any case.