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
``plt.show`` indeed isn't necessary on my local build, but for the ones on docs.scipy.org it does seem to be. May depend on the version of the plot-directive extension used.

``plt.show()`` is used in most docstring examples, I'll add it.