add note
the same note as in
@@ -30,6 +30,9 @@ macro_rules! iprintln {
/// `None` variant the caller must ensure that the macro is called from a function that's executed
/// at most once in the whole lifetime of the program.
///
+/// # Note
+/// This macro is unsound on multi core systems.
+///
/// # Example
///
/// ``` no_run
