users guide: Release notes for exception backtrace work

docs/users_guide/9.8.1-notes.rst

@@ -228,6 +228,17 @@ Runtime system
 
 - ``Data.Tuple`` now exports ``getSolo :: Solo a -> a``.
 
+- Exceptions can now carry arbitrary user-defined annotations via the new
+  :base-ref:`GHC.Exception.Type.ExceptionContext` implicit parameter of
+  ``SomeException``. These annotations are intended to be used to carry
+  context describing the provenance of an exception.
+
+- GHC now collects backtraces for synchronous exceptions. These are carried by
+  the exception via the ``ExceptionContext`` mechanism described above.
+  GHC supports several mechanisms by which backtraces can be collected which
+  can be individually enabled and disabled via
+  :base-ref:`GHC.Exception.Backtrace.setEnabledBacktraceMechanisms`.
+
 ``ghc-prim`` library
 ~~~~~~~~~~~~~~~~~~~~