Documentation should state which field changes will Force New

[displague]

Looking at the documentation, it is not clear if, for example, changing the metal_device user_data will cause the device to be recreated of if that user_data change will only update the metadata after the initial configuration. (relates to packethost/terraform-provider-packet#236)

Regardless of that situation, all ForceNew fields should include documentation that changes to this field will recreate the resource, with any special notes added as needed.

In the case of userdata, (I don't recall how it is treated), it might be expected for ForceNew behavior to exist. So we should also document cases where ForceNew may be expected, but this is not the case (and why).