They are briefly described in the manual and elsewhere on the internet. I think the older manual had a little more description, but I could be wrong. Basically the selected waveform is an LFO affecting the selected parameters by the amount selected. The waveforms themselves are just the basics and then some various takes on random, pulse, ramp, and step-type waveforms.
Almost every synth uses this same idea, although it is usually refered to as an LFO rather than "modulation". The interesting waveforms struck me as different at the time it first came about, but there are other synths that use this same sort of scenario (LFOs that aren't "regular waveforms like sine, triangle, square, saw, etc.) Although the ones I am thinking of aren't using granular synthesis so the whole index/shift/motion are sort of unique parameters. As far as the waveforms go, some other synths let you draw your own modulation waveforms or combine pieces of other waveforms, or shift waveforms in the time domain. If anything the Malstrom is very limiting in that you can only modulate the parameters they give you (not true if you flip the rack and use CV, then you can modulate anything) and with "only" 32 modulation waveforms(not including modulating "A" with "B" (one of my faves)), but I have always loved the simplicity of the Malstrom and how it still produces crazy-ass sounds.