Comment the relevant line where it seems things should fail

I don't understand why what I've done works. This comment is meant to simply highlight the
line in the code that seems to be the cause of boot protocol failing without the earlier
changes I made (i.e. `boot_protocol_`).

src/BootKeyboard/BootKeyboard.cpp

@@ -153,6 +153,11 @@ bool BootKeyboard_::setup(USBSetup& setup) {
     if (request == HID_GET_PROTOCOL) {
       // TODO improve
 #ifdef __AVR__
+      // This is where the `protocol` variable is used to set...something, but I don't
+      // know what. With the change that I've made to use `boot_protocol_` instead, this
+      // would always be set to `HID_REPORT_PROTOCOL`, even when sending boot protocol
+      // reports (successfully). It doesn't seem correct, but it works on macOS and Linux
+      // (or at least, my old Ubuntu machine).
       UEDATX = protocol;
 #endif
 #ifdef ARDUINO_ARCH_SAM