Add documentation for DefaultViewConverter's  `shape` parameter

There was no documentation and I had to take a couple minutes to read the code and figure out why my data, which was not in the `('b', 0, 1, 'c')` format, was messed-up when converting it in topological view: I though that `shape` needed to follow the same order as `axes`.

I added this small bit of documentation so that it's easier to determine which format `shape` expects.