add comment about panic handler

crates/tests/no_std/src/lib.rs

@@ -22,6 +22,8 @@ fn _test() {
     let _ = windows::core::ComObject::new(App);
 }
 
+// This panic handler will cause a build error if an indirect `std` dependency exists as `std`
+// will include its own panic handler and conflict with this one.
 #[cfg_attr(not(test), panic_handler)]
 fn _panic(_: &core::panic::PanicInfo<'_>) -> ! {
     loop {}