Update flash-nspanel.md

Document that the screen is not changing after flashing the firmware
That way end users are aware

Author: Sebastien Vermeille

docs/getting-started/flash-nspanel.md

@@ -152,9 +152,12 @@ Then, flash the firmware with your tool of choice:
         port will be something similar to `/dev/ttyUSB0`.
         :::
      </TabItem>
-</Tabs>
 
+</Tabs>
 
+:::warning
+After the flash of the firmware, you will not see any visible change on the NSPanel display this is normal at this stage.
+:::