vmware_vm_shell: doc: add missing defaults

lib/ansible/modules/extras/cloud/vmware/vmware_vm_shell.py

@@ -38,11 +38,13 @@ options:
             - The datacenter hosting the VM
             - Will help speed up search
         required: False
+        default: None
     cluster:
         description:
             - The cluster hosting the VM
             - Will help speed up search
         required: False
+        default: None
     vm_id:
         description:
             - The identification for the VM
@@ -57,14 +59,17 @@ options:
             - 'inventory_path'
             - 'vm_name'
         required: False
+        default: None
     vm_username:
         description:
             - The user to connect to the VM.
         required: False
+        default: None
     vm_password:
         description:
             - The password used to login to the VM.
         required: False
+        default: None
     vm_shell:
         description:
             - The absolute path to the program to start. On Linux this is executed via bash.