pw.8: Improve argument names

Arguments passed to flags like -d and -k had ambiguous names like "dir".
Change that to more descriptive names like "homedir".

Also, clarify that -u min,max is for UIDs and -i min,max for GIDs.

MFC after: 1 week

(cherry picked from commit feb04c7b7c91a7042dd123773693e9f71114c3c8)