particle-{argon,boron}: make note on untested nature of board_bat_voltage for this board

Author: miri64

boards/particle-argon/doc.txt

@@ -18,6 +18,12 @@ that provides access to multiple communication protocols: BLE, 802.15.4 and WiFi
 
 The board datasheet is available [here](https://docs.particle.io/assets/pdfs/datasheets/argon-datasheet.pdf)
 
+@experimental Support to measure the voltage via ADC using the
+`board_bat_voltage` feature (see [Feature List](@ref feature-list)) is provided
+for this board. However, it was only ever tested for the very similar
+@ref boards_particle-xenon. If you encounter any errors when measuring the
+voltage for the Particle Argon, please report this!
+
 ### Flash the board
 
 See the `Flashing` section in @ref boards_common_particle-mesh.

boards/particle-boron/doc.txt

@@ -18,6 +18,12 @@ that provides access to multiple communication protocols: BLE, 802.15.4 and LTE.
 
 The board datasheet is available [here](https://docs.particle.io/assets/pdfs/datasheets/boron-datasheet.pdf)
 
+@experimental Support to measure the voltage via ADC using the
+`board_bat_voltage` feature (see [Feature List](@ref feature-list)) is provided
+for this board. However, it was only ever tested for the very similar
+@ref boards_particle-xenon. If you encounter any errors when measuring the
+voltage for the Particle Boron, please report this!
+
 ### Flash the board
 
 See the `Flashing` section in @ref boards_common_particle-mesh.