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

I think this will not render well in html (one new line in rst is ignored, so the three lines will be appended to each other in html). Possible solutions: use a literal block (http://docutils.sourceforge.net/docs/user/rst/quickref.html#literal-blocks), or make a table of it (http://docutils.sourceforge.net/docs/user/rst/quickref.html#tables), or maybe simpler just make a list of it.