rc.conf.5: Improve NOAUTO configuration + tag SPDX
- Show how to start a NOAUTOed interface
- "configured" is not quite right, try to improve that
MFC after: 3 days
Event: EuroBSDcon Devsummit 2026
Reviewed by: adrian
Discussed with: Antranig Vartanian <antranigv@freebsd.am>
Differential Revision: https://reviews.freebsd.org/D59571