Document sudo_user

rst/playbooks.rst

@@ -79,6 +79,7 @@ Support for running things from sudo is also available::
     - hosts: webservers
       user: yourname
       sudo: True
+      sudo_user: privileged_user
 
 If you need to specify a password to sudo, run `ansible-playbook` with ``--ask-sudo-pass`` (`-K`).
 If you run a sudo playbook and the playbook seems to hang, it's probably stuck at the sudo prompt.