Correct me if I'm wrong, but this article, entitled 'Associating a device or computer to your Apple ID' simply gives instructions on how to remove a device. It barely mentions associating a device at all. Perhaps this is why it has such a low percentage of people who found it helpful, something Apple seem to have overlooked.
When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID. This article describes how to manage these associations.
Though that may not always associate a computer or device. If you have comments about the article then you could leave feedback for Apple : http://www.apple.com/feedback/
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