Add some doc clarifications for the test framework

src/test_utils.rs

@@ -10,6 +10,9 @@ fn panic(info: &PanicInfo) -> ! {
     loop {}
 }
 
+/// This function is hooked up to the #![test_runner] attribute made possible by the
+/// https://doc.rust-lang.org/unstable-book/language-features/custom-test-frameworks.html unstable
+/// feature. cf. https://os.phil-opp.com/testing/
 pub fn test_runner(tests: &[&dyn TestFunction]) {
     serial_println!("Running {} test(s)", tests.len());
     for test in tests {
@@ -34,6 +37,7 @@ where
     }
 }
 
+/// Exits qemu by writing to a special qemu emulated port.
 pub fn exit_qemu(exit_code: QemuExitCode) {
     use x86_64::instructions::port::Port;
     unsafe {