Some minor fixes for stats.wishart addition.

- fixes to docstrings (references, see also, added in 0.16.0)
- use ``np.linalg.slogdet`` where appropriate
- group tests for multivariate distributions
I do the same, ``make html`` is the easiest. I don't have ``numpy/doc/sphinxext`` in my path but have simply installed ``numpydoc``.