???: embedded comments into volume_mask_dict for TODO (+ minor code and DOC changes)

I thought to figure out the reason for https://github.com/PyMVPA/PyMVPA/issues/153 (which was something else -- addressed now), so start to go through the code with feeling that interface could be improved.

In this PR we could discuss those TODOs I have embedded straight on this code review.   If you do not see principal problems and would like to address them -- just send me a PR against this branch on @yarikoptic, or I could even push it to our stock repository...