DOC: update df.as_matrix and df.values.  Closes gh7413

I see the value in the ``See Also`` from ``as_matrix`` to ``values``, but, I don't know as the converse makes sense.  ``as_matrix`` points to ``values`` because ``values`` is what is actually returned.  Adding a reference in this direction (with a word of caution) seems to be giving a tool, and explicitly telling people not to use it.