[macro_util] Add comment to core_reexport (#586)

* [macro_util] Add comment to core_reexport

Add a comment explaining why we don't change core_reexport
to `pub use core as core_reexport`.

* [macro_util] Clarify comment on core_reexport

src/macro_util.rs

@@ -397,6 +397,11 @@ pub unsafe fn transmute_mut<'dst, 'src: 'dst, Src: 'src, Dst: 'dst>(
     unsafe { &mut *dst }
 }
 
+// NOTE: We can't change this to a `pub use core as core_reexport` until [1] is
+// fixed or we update to a semver-breaking version (as of this writing, 0.8.0)
+// on the `main` branch.
+//
+// [1] https://github.com/obi1kenobi/cargo-semver-checks/issues/573
 pub mod core_reexport {
     pub use core::*;