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.
> The other alternative, if you know the base class name,
> is just ndarray.squeeze(self, ...) as ndarray is already known
> in the matrix class as it inherits fromit.

I'm not sure I follow you here, but changing `N.ndarray.squeeze(self, axis=axis)` to `ndarray.squeeze(self, axis=axis)` causes a `NameError` because `ndarray` is not defined.  numpy.matrix actually inherits from `N.ndarray`.