Document the risk of actions getting replayed in FixedUpdate

[cBournhonesque]

If we have a system in FixedUpdate that calls `buffer_message_to_send, then during each rollback the system would get called multiple times again, which means the message would be sent multiple times.

Maybe we should provide a schedule FixedUpdateWithoutRollback for actions that should happen in FixedUpdate but shouldn't be rolled back?