[Docs] Clarification of using become with delegation

docs/ansible_detailed.rst

@@ -320,6 +320,8 @@ container.
 To enable connection delegation, set ``mitogen_via=<inventory name>`` on the
 command line, or as host and group variables.
 
+Note: the complete form of ``mitogen_via`` is ``[[become_method:]become_user@]inventory_hostname``. If using a password, it must be specified either in ``ansible_become_pass`` or ``ansible_become_password`` of the bastion.
+
 .. code-block:: ini
 
     # Docker container on web1.dc1 is reachable via web1.dc1.