fix comment

nexus/src/app/instance.rs

@@ -411,10 +411,15 @@ impl super::Nexus {
         // Users are allowed to request a start or stop even if the instance is
         // already in the desired state (or moving to it), and we will issue a
         // request to the SA to make the state change in these cases in case the
-        // runtime state we saw here was stale.  However, users are not allowed
-        // to change the state of an instance that's migrating, failed or
-        // destroyed, except that multiple requests to migrate in are allowed
-        // for idempotency.
+        // runtime state we saw here was stale.
+        //
+        // Users cannot change the state of a failed or destroyed instance.
+        // TODO(#2825): Failed instances should be allowed to stop.
+        //
+        // Migrating instances can't change state until they're done migrating,
+        // but for idempotency, a request to make an incarnation of an instance
+        // into a migration target is allowed if the incarnation is already a
+        // migration target.
         let allowed = match runtime.run_state {
             InstanceState::Creating => true,
             InstanceState::Starting => true,