better comments around version checking

service/r2/store/kv/store.go

@@ -197,8 +197,15 @@ func (s *store) UpdateRuleSet(
 	if err != nil {
 		return nil, handleUpstreamError(err)
 	}
+	// If ruleset version fetched from KV matches the change set version, the check and set operation will succeed if the underlying
+	// KV version remains the same before the change is committed and fail otherwise. If the fetched version doesn't
+	// match, we will fail fast.
 	if version != rs.Version() {
-		return nil, r2.NewConflictError("mismatched ruleset version number")
+		return nil, r2.NewConflictError(fmt.Sprintf(
+			"ruleset version mismatch: current version=%d, expected version=%d",
+			rs.Version(),
+			version,
+		))
 	}
 
 	mutable := rs.ToMutableRuleSet().Clone()