Clarify that a dataset spec defines the specs for an n-dimensional array

[rly]

It is not totally clear that a dataset spec defines the specs for an n-dimensional array, and that the corresponding API class for a dataset type must include an array. See discussion in NeurodataWithoutBorders/nwb-schema#623

[h-mayorquin]

Thanks Ryan for tracking this. To add some more context

There is a comment that clarifies that the dataset is meant to represent an n-dimensional array:

But as for the second point:

that the corresponding API class for a dataset type must include an array.

This requirement I could not find.