Add docs

Author: avsaase

src/sdcard/sd_card_device.rs

@@ -155,6 +155,8 @@ where
     }
 }
 
+/// Perform a transaction against the device. This sends a dummy `0xFF` byte to the device after
+/// deasserting the CS pin but before unlocking the bus.
 fn bus_transaction<BUS, CS>(
     bus: &mut BUS,
     cs: &mut CS,
@@ -195,6 +197,7 @@ where
     Ok(())
 }
 
+/// Send 80 clock pulses to the device with CS deasserted. This is needed to initialize the SD card.
 fn send_clock_pulses<BUS, CS>(bus: &mut BUS, cs: &mut CS) -> Result<(), SdCardDeviceError>
 where
     BUS: SpiBus,