docs(plugin): add manual validation guidance for MOTD check (FIREWALL-003)

[unclesp1d3r]

Summary

FIREWALL-003 (Message of the Day) always returns UNKNOWN because MOTD is an OS-level file (/etc/motd), not stored in config.xml.

Suggested Manual Validation Steps

1. SSH into the firewall: ssh admin@<firewall-ip>
2. Check MOTD content: cat /etc/motd
3. Verify it contains appropriate legal notice / system identification
4. Check if motd is regenerated on reboot (may be overwritten by OPNsense)

Related

• feat(plugin): report compliant controls alongside findings in blue mode reports #456 — parent compliance feature
• docs(plugin): add manual validation guidance for SSH banner check (FIREWALL-001) #504 — SSH banner manual guidance (same pattern)