Improve docstrings in group api
Improve docstrings in group api

sutartmelson/girder,jcfr/girder,adsorensen/girder,chrismattmann/girder,manthey/girder,RafaelPalomar/girder,girder/girder,kotfic/girder,manthey/girder,data-exp-lab/girder,jcfr/girder,RafaelPalomar/girder,Xarthisius/girder,RafaelPalomar/girder,girder/girder,kotfic/girder,RafaelPalomar/girder,essamjoubori/girder,kotfic/girder,essamjoubori/girder,jcfr/girder,salamb/girder,jcfr/girder,msmolens/girder,Kitware/girder,opadron/girder,Kitware/girder,chrismattmann/girder,Xarthisius/girder,kotfic/girder,manthey/girder,RafaelPalomar/girder,data-exp-lab/girder,jbeezley/girder,kotfic/girder,sutartmelson/girder,adsorensen/girder,manthey/girder,jbeezley/girder,msmolens/girder,opadron/girder,salamb/girder,msmolens/girder,salamb/girder,Xarthisius/girder,adsorensen/girder,Xarthisius/girder,jbeezley/girder,girder/girder,data-exp-lab/girder,adsorensen/girder,essamjoubori/girder,chrismattmann/girder,salamb/girder,sutartmelson/girder,Kitware/girder,Kitware/girder,sutartmelson/girder,adsorensen/girder,opadron/girder,jcfr/girder,chrismattmann/girder,data-exp-lab/girder,essamjoubori/girder,data-exp-lab/girder,salamb/girder,msmolens/girder,girder/girder,Xarthisius/girder,sutartmelson/girder,jbeezley/girder,essamjoubori/girder,msmolens/girder,chrismattmann/girder,opadron/girder,opadron/girder