Add comment on macro hygenics bug

Co-authored-by: Jubilee <[email protected]>

Author: ChrisDenton

library/std/src/sys/pal/windows/api.rs

@@ -48,7 +48,9 @@ macro_rules! wide_str {
 
 /// Creates a UTF-16 string from a str without null termination.
 macro_rules! utf16 {
-    // Note: this macro uses triple underscores to avoid const cycles
+    // Note: this macro uses triple underscores to avoid const cycles which,
+    // despite macro hygienics, occur if we use a const with the same name
+    // as an argument to the macro, see issue rust-lang/rust#123691
     ($str:expr) => {{
         const ___UTF8: &str = $str;
         const ___UTF16_LEN: usize = crate::sys::pal::windows::api::utf16_len(___UTF8);