Harden change-notification guidance

[msporny]

From the SING review:

2. Change-notification hardening – emphasized the attack surface (spoofing, DoS, linkability) and need for authenticated, rate-limited channels.

• Notification channels MUST authenticate their sources and integrity-protect messages.
• Implementers SHOULD bind notifications to DID state deltas (prev-hash → new-hash + timestamp) and apply rate limits.
• Specs SHOULD explicitly call out that third-party or aggregated channels present the risk of spoofing, DoS, or linkability.

[msporny]

A PR should be raised that identifies Security Recommendations to harden change notifications.