rnt@1e2031665aec7ab8b939ecab7f0b9a3dc85ad769
manpage: fixed a variable name in the example
manpage: option descriptions use bsu_* more

an option description mentioning other values refers to them
through the config file variable names, to make the fact that
they can come from .bs-update more readily apparent.
manpage: option descriptions mention defaults

excepting -h and -n.