DOC: matrix.squeeze docstring

Add a docstring that should be less wrong than http://docs.scipy.org/doc/numpy/reference/generated/numpy.matrix.squeeze.html and more informative.  The behavior should not be affected.
> Actually, sounds like an argument for super ;)

You mean that it would make more sense to return an ndarray than a matrix?  This seems like more than documentation so I hope it would be out of scope for this PR...