audit_warn: Avoid pointless fork and path search

[des]

Summary

While here, document what the script actually does, not what it once did
on a project branch somewhere that was never merged.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Qualys, Inc.

[ziaee]

contrib/openbsm/man/audit_warn.5

51

While here, macros should not be in list width specifiers. It is an undefined behavior that at best renders as zero spaces but sometimes renders as -1 space.