[clang] Revise relnote for int->enum conversion.

Include more specifics from recent discussion on llvm#143034.

Author: efriedma-quic

clang/docs/ReleaseNotes.rst

@@ -66,9 +66,10 @@ C++ Specific Potentially Breaking Changes
 - A workaround for libstdc++4.7 has been removed. Note that 4.8.3 remains the oldest
   supported libstdc++ version.
 - Added ``!nonnull/!align`` metadata to load of references for better codegen.
-- Checking for int->enum conversions in constant expressions is more strict;
-  in particular, ``const E x = (E)-1;`` is not treated as a constant if it's
-  out of range. This impacts old versions of Boost.  (#GH143034)
+- Checking for integer to enum conversions in constant expressions is more
+  strict; in particular, ``const E x = (E)-1;`` is not treated as a constant
+  if it's out of range. The Boost numeric_conversion library is impacted by
+  this; it was fixed in Boost 1.81. (#GH143034)
 
 ABI Changes in This Version
 ---------------------------