Add support for max pods per node to node pool. (hashicorp#2038)

Author: nat-henderson

docs/r/container_node_pool.html.markdown

@@ -129,6 +129,10 @@ Note: You must be provide region for regional clusters and zone for zonal cluste
 * `management` - (Optional) Node management configuration, wherein auto-repair and
     auto-upgrade is configured. Structure is documented below.
 
+* `max_pods_per_node` - (Optional) The maximum number of pods per node in this node pool.
+    Note that this does not work on node pools which are "route-based" - that is, node
+    pools belonging to clusters that do not have IP Aliasing enabled.
+
 * `name` - (Optional) The name of the node pool. If left blank, Terraform will
     auto-generate a unique name.