Document the MatrixTypes module

[alpmestan]

The MatrixTypes module could benefit from some documentation, particularly the arguments to the GADT constructors.

[cartazio]

yup, theres plans. stay tuned

[cartazio]

its going to have a few more blas/lapack matrix formats in there soon. I'm actually planning to factor it out into its own package soon

[cartazio]

@alpmestan it'd help if you asked what you were confused by.

hopenblas is really meant to be a bare bones binding that is used by a library author to evade having to deal with any ffi binding work themselves. Any utility beyond that is a happy accident