Update container_cluster.html.markdown (#1570)

* Update container_cluster.html.markdown

Just a more clear explanation of what happens when this field is not provided, since there was already an issue with this topic.

* Update container_cluster.html.markdown

Cleared extra comma.

Author: brunoban

website/docs/r/container_cluster.html.markdown

@@ -242,7 +242,7 @@ The `master_auth` block supports:
     the Kubernetes master endpoint
 
 If this block is provided and both `username` and `password` are empty, basic authentication will be disabled.
-This block also contains several computed attributes, documented below.
+This block also contains several computed attributes, documented below. If this block is not provided, GKE will generate a password for you with the username `admin`.
 
 The `master_authorized_networks_config` block supports: