helper/schema: Resouce.Diff no longer ResourceProvider API compatible
helper/schema: Resouce.Diff no longer ResourceProvider API compatible

The old comments said that this interface was API compatible with
terraform.ResourceProvider's Diff method - with the addition of passing
down meta to it, this is no longer the case.

Not too sure if this is really a big deal - schema.Resource never fully
implemented terraform.ResourceProvider, from what I can see, and the
path from Provdier.Diff to Resource.Diff is still pretty clear. Just
wanted to remove an outdated comment.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk