add docstring with config info for kannel backend
add docstring with config info for kannel backend

catalpainternational/rapidsms,lsgunth/rapidsms,ehealthafrica-ci/rapidsms,eHealthAfrica/rapidsms,unicefuganda/edtrac,eHealthAfrica/rapidsms,lsgunth/rapidsms,unicefuganda/edtrac,catalpainternational/rapidsms,catalpainternational/rapidsms,peterayeni/rapidsms,eHealthAfrica/rapidsms,peterayeni/rapidsms,peterayeni/rapidsms,peterayeni/rapidsms,catalpainternational/rapidsms,dimagi/rapidsms,caktus/rapidsms,ehealthafrica-ci/rapidsms,dimagi/rapidsms,caktus/rapidsms,lsgunth/rapidsms,unicefuganda/edtrac,caktus/rapidsms,ehealthafrica-ci/rapidsms,lsgunth/rapidsms