suzerain: document file for common details

Will more or less want the same description for the packc/packf
routines, so move it to a commonly-available space.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43862 0bd85f0b-c151-43b9-a1e9-27231104551f