Add a non-trivial decorator example to class-based views docs

[Dan McGee]

To save someone the hassle I just had trying to go beyond the all-too-simple @login_required example. This seemed to work for me at least, so I hope it is correct:

    (r'^delete/(?P<pk>\d+)/$',
        permission_required('main.delete_todolist')(DeleteTodolist.as_view())),

Attached a very simple suggestion for the docs, feel free to redo it if it is not in the style but I tried to look at other relevant examples.

[Dan McGee]

Possible documentation update

[Julien Phalip]

That's great, thank you!

[Simon Meers]

In [16534]:

Fixed #15715 -- added non-trivial decorator example to CBV docs. Thanks toofishes.

[Simon Meers]

In [16535]:

[1.3.X] Fixed #15715 -- added non-trivial decorator example to CBV docs. Thanks toofishes.

Backport of r16534 from trunk.