X.A.TopicSpace: Update documentation
X.A.TopicSpace: Update documentation

  - Only suggest the usage of TopicItem; this is much easier to work
    with and essentially a straight upgrade to the old method.

  - Use a more stripped-down example so we don't confuse beginners more
    than necessary (though this is still not optimal).

xmonad/xmonad-contrib,xmonad/xmonad-contrib