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 `make html-scipyorg` and I do seem to need `plt.plot()`. Not sure what is different between the two.

This is surely a rather minor point, but all else being equal I'd rather have it implicit in docstrings (ditto for `import matplotlib.pyplot as plt`).

In any case, it's OT for this PR; my original comment is moot.