boards: arm: add board revision to board description

add board revision number to board description (MB1319C)

Signed-off-by: Guðni Már Gilbert <gudni.m.g@gmail.com>

Author: gudnimg

boards/arm/nucleo_l412rb_p/doc/index.rst

@@ -127,6 +127,8 @@ Nucleo L412RB-P provides the following hardware components:
 - Development support: serial wire debug (SWD), JTAG, Embedded Trace Macrocell |trade|
 - All packages are ECOPACK2 compliant
 
+Note: the current board revision is C. (MB1319C)
+
 More information about STM32L412RB can be found here:
 
 - `STM32L412RB on www.st.com`_
@@ -191,6 +193,8 @@ Default Zephyr Peripheral Mapping:
 - USER_PB   : PC13
 - LD4 : PB13
 
+Note: SPI2 CS pin (PB12) is not located on the Arduino connector.
+
 System Clock
 ------------