docs: update design/docs/service.mdx for PR MystenLabs#568

Automated update based on: New SignCommitteeTransition RPC added where peers sign with historical epoch BLS keys. Leader now drives guardian committee handoff to match on-chain epoch.

Author: jessiemongeon1

design/docs/service.mdx

@@ -9,6 +9,31 @@ Hashi node exposes an HTTP service, secured by Transport Layer Security (TLS)
 using a self-signed cert (the ed25519 public key is available in the Hashi
 System State object), and serves a gRPC `HashiService`.
 
+## HashiService RPCs
+
+The `HashiService` gRPC interface exposes the following RPCs:
+
+- **SignCommitteeTransition:** Peers sign committee transition messages using their historical epoch BLS keys. The leader invokes this RPC on other committee members to collect signatures needed to advance the guardian committee to match the current on-chain epoch.
+
+## Committee transitions
+
+When the on-chain epoch changes, the guardian committee must transition to
+reflect the updated committee composition. The leader drives this handoff
+process:
+
+1. The leader detects that the on-chain epoch has advanced beyond the current
+   guardian committee epoch.
+2. The leader calls `SignCommitteeTransition` on each peer, requesting a
+   signature over the new committee state.
+3. Each peer signs the transition message using its historical epoch BLS key
+   that corresponds to the epoch in which the transition originates.
+4. Once the leader collects enough signatures, it finalizes the guardian
+   committee handoff so that the guardian committee matches the on-chain epoch.
+
+This mechanism ensures that the guardian committee stays in sync with the
+on-chain committee state and that transitions are authenticated by the members
+of the originating epoch.
+
 ## Sui contracts
 
 - The Hashi Move packages are published as normal packages. The Hashi packages
@@ -43,4 +68,4 @@ block-beta
   end
 
   broadcast["Ordered broadcast channel"]
-```
+```