[O] Showing DONE tasks in agenda?

[Peter Davis]

When I display my TODO items (with C-c a t), there's a line at the top 
of the buffer that says:

Available with `N r': (0)[ALL] (1)TODO (2)DONE

but I don't know what this means. How can I display all (TODO, DONE, 
whatever) items?

Typing 'N r' in the buffer doesn't do it.

Thanks,
-pd