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.
No, but I'd be curious as to why it behaves differently in the two cases. AFAICT, the same function should be called, but it looks like `ndarray.__new__` is called instead of `matrix.__new__`, or something like that. Hmm...