sudorule: add SELinux transition examples to plugin doc
    
    It is not obvious how to add SELinux type and role transitions to a
    Sudo rule.  Update the 'sudorule' plugin documentation with examples
    of how to do this.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/3461
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>