Clarify how initial runtime states work

nexus/src/app/instance.rs

@@ -356,11 +356,14 @@ impl super::Nexus {
         // target sled agent will populate its instance manager with the
         // contents of this modified record, and that record needs to allow a
         // transition to the Starting state.
-        let new_runtime = nexus_db_model::InstanceRuntimeState {
+        //
+        // If the instance does exist on this sled, this initial runtime state
+        // is ignored.
+        let initial_runtime = nexus_db_model::InstanceRuntimeState {
             state: nexus_db_model::InstanceState(InstanceState::Creating),
             ..db_instance.runtime_state
         };
-        db_instance.runtime_state = new_runtime;
+        db_instance.runtime_state = initial_runtime;
         self.instance_ensure_registered(opctx, &authz_instance, &db_instance)
             .await?;