docs: record the FR-3 decision, an operator-set outage traffic allowance

[timwu20]

Supersedes #141, the original proposal, which is closed as outdated. Records what was decided on 2026-10-05 for FR-3 in the design note:

• During an outage, the operator sets an outage traffic allowance in the sync operator app's local config, following the runbook, and removes it once the global synchronizer is back. While it's set, the app raises each member's limit to its purchased total plus the allowance; once it's removed, the app sets each limit back to the purchased total.
• The app warns while the allowance is set, when a purchase lands while it's still set, and when a member's limit doesn't come back to its purchased total after removal.
• Nothing detects the outage automatically and there is no Daml change, so it all ships in 0.10.3. feat: govern an outage traffic advance on the registration canton-network/splice-multi-sync#61 and feat: grant the outage traffic advance and take it back canton-network/splice-multi-sync#66 are closed in its favour.

It's implemented in canton-network/splice-multi-sync#71. The earlier options are under "Alternatives considered". Refs #129.