Add documentation

i2c2-target/src/lib.rs

@@ -150,6 +150,10 @@ impl I2C2 {
         }
     }
 
+    /// Install a callback which will be called whenever an I²C
+    /// request arrives. This callback will decide which bytes are
+    /// sent back if a read request follows. The read operation will
+    /// be completed by zeroes as needed.
     pub fn set_callback(callback: Callback) {
         unsafe { STATE.callback = Some(callback) };
     }