That is old information, "disked" MDs are no longer supported in 0710. There were stability and synchonisation issues with it, so it was removed as a feature. It is likely that it will be added back in at some point, but not yet. It is possible to hack one to be disked rather than diskless, but for these reasons it is highly inadvisable. I have removed that section from the wiki to prevent confusion... if you find any other references to this, could you please send them to me so that I can correct them?
In the meantime, the answer for you is just to configure your BIOS to PXE/Network boot, and turn it on. This will trigger the core to set it up as a diskless MD. These work very well and are easy to set up. The only issue you may encounter is, if you have already started to set this PC up and the core has seen it, it may recognise it by MAC address and not trigger the auto-setup process. If this is the case, you should see an MD device in the Devices Tree on the web admin console. You can delete this device and reload the router, then reboot the MD PC again and it should trigger the set up.