Doc: update docstring of `get_user_shell()`
Doc: update docstring of `get_user_shell()`

cvmfs/cvmfs,Moliholy/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,alhowaidi/cvmfsNDN,reneme/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,djw8605/cvmfs,alhowaidi/cvmfsNDN,alhowaidi/cvmfsNDN,cvmfs/cvmfs,trshaffer/cvmfs,trshaffer/cvmfs,DrDaveD/cvmfs,alhowaidi/cvmfsNDN,reneme/cvmfs,DrDaveD/cvmfs,reneme/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,reneme/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,Gangbiao/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,reneme/cvmfs,djw8605/cvmfs,Moliholy/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,Moliholy/cvmfs,alhowaidi/cvmfsNDN,Moliholy/cvmfs,djw8605/cvmfs,trshaffer/cvmfs,Moliholy/cvmfs