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.
If you are suggesting to change the behavior, here's a relevant ancient discussion on the mailing list http://mail.scipy.org/pipermail/numpy-discussion/2005-October/005306.html.