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.

[msporny]

PR #916 has been raised to address this issue. This issue will be closed once PR #916 has been merged.

[msporny]

We ended up deleting the section because the WG felt like we had not done a proper analysis on a feature that might exist in the future. We didn't feel it was right to speak to something that we had not seen broad implementations of yet.