Document how other datatypes do or might fit into the picture

[rreusser]

Despite having worked with ndarrays, it's not quite clear to me exactly what the scope of, say, lazy ndarrays or rle-encoded ndarrays fit into the picture. (Currently, the blas-like and linear algebra methods I wrote only accept dense ndarrays.) It would be beneficial to document exactly where they do or should work, both for developers and for users.

[rreusser]

I understand now: it's generic arrays.