Document major differences between QSIPrep and sMRIPrep/fMRIPrep

[tsalo]

For example, if you specify an output space in sMRIPrep you expect to get imaging files in that output space, but in QSIPrep you get outputs in T1w-ACPC space no matter what, and the output space you request just produces a transform (which then gets used in QSIRecon). There are a few differences like this that I think could be enumerated in the documentation for folks who are more familiar with the other 'preps.