node.instance_uuid can not be replaced by JSON PATCH "add" operator

The RFC for a JSON PATCH operation specifies that the "add" operator should also replace existing values. This works for most of Ironic's API. However, it is prevented from working on the node.instance_uuid property by this section in db.sqlalchemy.api

https://github.com/openstack/ironic/blob/master/ironic/db/sqlalchemy/api.py#L373

    def update_node(self, node_id, values):
...
            # Prevent instance_uuid overwriting
            if values.get("instance_uuid") and ref.instance_uuid:
                raise exception.NodeAssociated(node=node_id,
                                instance=ref.instance_uuid)

The reason for this is clear: prevent a race whereby two nova-compute agents might try to associate the same node to different Nova instances, by serializing this process at Ironic's database (on a per-node level, not globally).

However, this is not obvious from the API itself, or from our API documentation.

We should document that the node.instance_uuid property does not adhere to the JSON PATCH RFC spec.