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.
> Does super work then?

It causes a `test_instance_methods` assertion to fail in testing.  I assume this causes the matrix member function to return an ndarray instead of a matrix, but I haven't looked into it.  Should I look into this further before the documentation PR is merged?