Suggest better docs for setSustainBegin and End

[DoctorNerve]

I think it would be helpful to document SequentialDataCommon
setSustainBegin better (I don't believe there's any javadoc comment for
it currently). I'd even suggest a simple diagram of, say a four frame
breakpoint envelope with the frames labelled 0, 1, 2, 3 and arrows
showing from where to where it'll loop based on setSustainBegin and
setSustainEnd. Same from setReleaseBegin/End

[philburk]

I added some Javadocs to setSustainBegin and setSustainEnd(). I will add the diagram in the user guide.