draft: usage notes, configurable debos scratchsize, nonfree option/variable

Please recommend better terms; I am uncertain about the use of the word "image," w/re: "*fs" (roofs, installfs), and then/also, "installer [image]" ...

I believe the terms used for the usage notes are very accurate, though some specific detail is lacking (acceptable values/lists). More could be done here, but these changes all are or seemed useful.