added comments

21-shell/drivers/keyboard.c

@@ -31,10 +31,11 @@ static void keyboard_callback(registers_t regs) {
         kprint_backspace();
     } else if (scancode == ENTER) {
         kprint("\n");
-        user_input(key_buffer);
+        user_input(key_buffer); /* kernel-controlled function */
         key_buffer[0] = '\0';
     } else {
         char letter = sc_ascii[(int)scancode];
+        /* Remember that kprint only accepts char[] */
         char str[2] = {letter, '\0'};
         append(key_buffer, letter);
         kprint(str);