Add more notes.

src/bare-metal/microcontrollers/examples/src/bin/hal.rs

@@ -29,7 +29,7 @@ use nrf52833_hal::{
 fn main() -> ! {
     let p = Peripherals::take().unwrap();
 
-    // GPIO port 0.
+    // Create HAL wrapper for GPIO port 0.
     let gpio0 = p0::Parts::new(p.P0);
 
     // Configure GPIO 0 pins 21 and 28 as push-pull outputs.

src/bare-metal/microcontrollers/mmio.md

@@ -9,6 +9,8 @@ micro:bit:
 
 <details>
 
+* GPIO 0 pin 21 is connected to the first column of the LED matrix, and pin 28 to the first row.
+
 Run the example with:
 
 ```sh