Explain intent of update_types

We think we will allow HMRC to set the update type when they publish a new manual/manual section. If we are going to do that, we need to be able to document what the values are for, so it makes sense to clarify what the values are intended for within content-store.
Can we be less ambiguous here? The word "might" isn't ideal for documentation. Unless we really don't know whether migrated content should be using the `republish` update type or not.