doc: add a reminder about field order for diff comparisons
doc: add a reminder about field order for diff comparisons

After spending hours re-learning this lesson more than once,
update the jobspec contribution guide with a reminder about
how to configure FieldDiff comparisons.

hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad