Document this more clearly higher up the function usage. Arguably it should default to Identity if doing "SyN only", but that would put it out of sync with antsRegistrationSyN.sh.
coverage: 85.061%. remained the same
when pulling 356e1350ad6e6f03ce15fa5bd27c4e61fef1c640 on init_transform_doc
into 2c624f7a52ab1d795f8a262b40967af90f9e1667 on master.
Document this more clearly higher up the function usage. Arguably it should default to Identity if doing "SyN only", but that would put it out of sync with antsRegistrationSyN.sh.