mitkDiffusion should have up-to-date and complete documentation.
This includes:
Tooltips for all buttons/sliders/interaction elements where there is even the slightest chance of not getting what it is about.
Actual user documentation, that can be accessed using 'F1' This will include checking and updating the old 3M diffusino documentation.
Documentation within the code ( see T9728 for that )
Please create sub-bugs for each perspective you are documenting.