Fix arguments in nodebalancer_node docs
Fix arguments in nodebalancer_node docs

* region argument does not exist
* label is optional
* nodebalancer_id & config_id are required

I believe that the list of exported attributes isn't quiet right, but I
was focusing on the incorrect parts.

btobolaski/terraform-provider-linode,btobolaski/terraform-provider-linode,btobolaski/terraform-provider-linode