Hi, To whomever maintains the manual...In Section 4.4( or 4.7?) - "Building a netboot image", at the top it should mention that the kernel you want to netboot MUST have CONFIG_ROOT_NFS and CONFIG_NFS_FS options built into kernel in order to boot. This will likely save people a lot of frustration, since these are not usually compiled in by default in many distros (not there in wheezy).
Thanks, -Bob