-bvalue_scaling: change valid choices to no, auto, yes.
-bvalue_scaling: change valid choices to no, auto, yes.

Also a few changes to corresponding inline documentation.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3