doc(sync): add a TODO about upgrading constance::sync::Mutex to use a real mutex

Author: yvt

src/constance/src/sync/mutex.rs

@@ -5,6 +5,9 @@ use crate::{
     prelude::*,
 };
 
+// TODO: Upgrade `Mutex` to use a real mutex. And then remove the paragraph
+//       in `Mutex` regarding this
+
 /// A mutual exclusion primitive useful for protecting shared data from
 /// concurrent access.
 ///