You can now find the latest docs here:
For Ext.nd based on ExtJS 3.x
http://extnd.github.com/SDK/docs/docs3/#!/api/Ext.nd
For Extnd based on ExtJS 4.x
http://extnd.github.com/SDK/docs/docs4/#!/api/Extnd
..and yes, as you can guess, Ext.nd is being re-done to work with ExtJS 4.x under the Extnd namespace instead of Ext.nd