doc: Add STM32H745I-DISCO RPTUN documentation

Signed-off-by: Nonpawit Ekburanawat <[email protected]>

Documentation/platforms/arm/stm32h7/boards/stm32h745i-disco/index.rst

@@ -3,9 +3,13 @@ ST STM32H745I-DISCO
 ===================
 
 Dual core support
------------------
+=================
 
 It is recommended to use CMake to build firmware consisting of multiple images.
+
+Dual instances
+--------------
+
 The commands to build ``nsh_xxx`` configurations are as follows::
 
   cmake -B build_cm7 -DBOARD_CONFIG=stm32h745i-disco:nsh_cm7 -GNinja
@@ -14,8 +18,19 @@ The commands to build ``nsh_xxx`` configurations are as follows::
   cmake --build build_cm7
   cmake --build build_cm4
 
+Dual core RPTun
+---------------
+
+The commands to build ``nsh_xxx_rptun`` configurations are as follows::
+
+  cmake -B build_cm7_rptun -DBOARD_CONFIG=stm32h745i-disco:nsh_cm7_rptun -GNinja
+  cmake -B build_cm4_rptun -DBOARD_CONFIG=stm32h745i-disco:nsh_cm4_rptun -GNinja
+
+  cmake --build build_cm7_rptun
+  cmake --build build_cm4_rptun
+
 Serial console
---------------
+==============
 
 The STM32H745I-DISCO board's ST-LINK interface is connected to USART3, which
 is used as the serial console for the Cortex-M7 core by default. The
@@ -28,7 +43,8 @@ USART3_RX         PB11
 USART3_TX         PB10
 ================= ====
 
-When using the nsh_xxx configuration, UART7 is assigned to the Cortex-M4 core.
+When using the ``nsh_xxx`` configuration, UART7 is assigned to the Cortex-M4
+core.
 The UART7 connections can be accessed via the Arduino connector on the board:
 
 ================= =================
@@ -40,4 +56,14 @@ UART7_TX          PB4 (Arduino D5)
 
 This allows the Cortex-M4 core to utilize a separate serial interface, making
 it possible to debug or interact with both cores simultaneously through
-different UART interfaces.
+different UART interfaces.
+
+RPTun
+-----
+
+When using the ``nsh_xxx_rptun`` configuration, users can connect to the
+CM4 core from the CM7 core by using the ``cu`` command::
+
+  nsh-cm7> cu -l /dev/ttyproxy
+  NuttShell (NSH) NuttX-10.4.0
+  nsh-cm4>