Update docs and variable types for worker node_labels

* Document worker pools `node_labels` variable to set the
initial node labels for a homogeneous set of workers
* Document `worker_node_labels` convenience variable to
set the initial node labels for default worker nodes

aws/container-linux/kubernetes/workers/variables.tf

@@ -106,7 +106,7 @@ variable "cluster_domain_suffix" {
 }
 
 variable "node_labels" {
-  description = "List of additional labels to add to worker nodes"
-  type = list
+  type = list(string)
+  description = "List of initial node labels"
   default = []
 }