boards/nrf52840dongle/doc: Update nrfutil pointers

Closes: RIOT-OS#19511

boards/nrf52840dongle/doc.txt

@@ -31,12 +31,15 @@ reset button as well as 15 configurable external pins.
 
 ### <a name=nrf52840dongle_flash> Flash the board </a>
 
-The board is flashed using its on-board boot loader; the
-[nrfutil](https://github.com/NordicSemiconductor/pc-nrfutil) program needs to
-be installed. That can turn the binary into a suitable zip file and send it to the
-bootloader.
+The board is flashed using its on-board boot loader; the proprietary
+[nrfutil](https://www.nordicsemi.com/Products/Development-tools/nRF-Util) program needs to
+be installed, and `nrfutil install nrf5sdk-tools` needs to be executed. Note
+that nrfutil, even when not running the "install" command, will install itself
+into `~/.nrfutil`. The older Python based version of nrfutil is no longer
+maintained by Nordic, and has become dysfunctional on Python 3.11.
 
-The process is automated in the usual `make flash` target.
+The nrfutil can turn the binary into a suitable zip file and send it to the
+bootloader. The process is automated in the usual `make flash` target.
 
 If RIOT is already running on the board, it will automatically reset the CPU and enter
 the bootloader.
@@ -45,13 +48,6 @@ manually by pressing the board's reset button.
 
 Readiness of the bootloader is indicated by LD2 pulsing in red.
 
-#### nrfutil installation
-
-On systems with Python 2 available, `pip install nrfutil` works.
-
-On systems with Python 3, a recent version of pip is required to install all dependencies;
-you may need to run `pip install --upgrade pip` before being able to run `pip install nrfutil` successfully.
-
 ### Accessing STDIO
 
 The usual way to obtain a console on this board is using an emulated USB serial port (CDC-ACM).