Add some clarification to NVME beta instructions

Add a bit more detail about running rpiboot from another PC
Fixes raspberrypi#1939

Author: peterharperuk

hardware/raspberrypi/bootmodes/nvme.md

@@ -21,13 +21,17 @@ To boot from NVMe you need pre-release versions of the bootloader, VideoCore fir
 
 ### Bootloader
 
-You need to use `rpiboot` to update the CM4 bootloader. Instructions for building `rpiboot` and configuring the IO board to switch the ROM to usbboot mode are in the [usbboot Github repository](https://github.com/raspberrypi/usbboot).
+You need to use `rpiboot` to update the CM4 bootloader to use the beta bootloader. If you are using a CM4 with an eMMC then you must also change the BOOT_ORDER to give NVMe priority, otherwise the CM4 will continue to boot from eMMC.
 
-If you are using a CM4 with an eMMC then you must change the BOOT_ORDER to give NVMe priority, otherwise the CM4 will continue to boot from eMMC. For example:
+Instructions for building `rpiboot` and configuring the IO board to switch the ROM to usbboot mode are in the [usbboot Github repository](https://github.com/raspberrypi/usbboot).
+
+To use `rpiboot` you need another PC (another Pi, PC or Mac), that is capable of building the `rpiboot` binary. This other PC is then connected to the CM4 IO board USB slave socket and you have to fit a jumper to the J2 header to disable eMMC boot. Then when `rpiboot` is executed it communicates with the CM4 to reprogram the eeprom. You can't change the eeprom on a CM4 from the device itself.
+
+For example run the following from another PC connected to the CM4 USB slave connector:
 
 ```
 cd usbboot/nvme
-sed -i 's/\(BOOT_ORDER=.*\)6\(.*\)/\1\26/' boot.conf
+sed -i'' 's/\(BOOT_ORDER=.*\)6\(.*\)/\1\26/' boot.conf
 ./update-pieeprom.sh
 cd ..
 ```
@@ -54,6 +58,8 @@ File read: pieeprom.bin
 Second stage boot server done
 ```
 
+Remember to disconnect USB from the slave socket AND remove the jumper from the J2 header when this is done.
+
 ### Firmware and kernel
 
 You must load pre-release versions of the VideoCore firmware and Raspberry Pi OS Linux kernel to the NVMe disk. When this software is officially released you can simply write the software to the NVMe disk directly with the Raspberry Pi Imager app; until then the easiest way to perform the update is: