boards/nrf52: update nrf52 common documentation

Add common flashing notes + move the doc in a separate doc.txt file

boards/common/nrf52/doc.txt

+/**
+@defgroup    boards_common_nrf52 nRF52 common
+@ingroup     boards_common
+@brief       Common files and configuration for nRF52 boards.
+
+## Flashing nrf52 boards
+
+By default, nrf52 boards are flashed using Jlink. @ref boards_nrf52840-mdk
+uses PyOCD by default.
+
+To flash the board, use `BOARD=<nrf52 board>` with the `make` command.<br/>
+Example with `hello-world` application:
+```
+    make BOARD=<nrf52 board> -C examples/hello-world flash
+```
+
+OpenOCD can also be used to flash nrf52 boards. For the moment, the latest
+stable version of OpenOCD (0.10) doesn't contain any support for nrf52 but
+versions built against the current development version can be used.
+
+To flash the board with OpenOCD, use the `PROGRAMMER` variable:
+```
+    PROGRAMMER=openocd make BOARD=<nrf52 board> -C examples/hello-world flash
+```
+
+*/

boards/common/nrf52/include/board_common.h

@@ -7,8 +7,7 @@
  */
 
 /**
- * @defgroup    boards_common_nrf52 NRF52 common
- * @ingroup     boards_common
+ * @ingroup     boards_common_nrf52
  * @brief       Shared files and configuration for all nRF52 boards.
  * @{
  *