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.
Reading about super, there are mixed reviews, but at least the given form should be compatible python 2 & 3 ;) 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. Another PR cleaning up all the current uses would be good. I guess one advantage of super, at least in the single inheritance case, is that you can change the base class. Not something we will be doing, however.