gh-101267: clarify the NEWS entry and use reST roles

Describe the actual failure (re-raising one shared exception appends a
traceback to it on each Future.result()) and cross-reference the API names.

Author: gpshead

Misc/NEWS.d/next/Library/2023-01-23-21-23-50.gh-issue-101267._f-cFH.rst

@@ -1 +1,7 @@
-When a worker process terminates unexpectedly, ``concurrent.futures.ProcessPoolExecutor`` no longer shares one ``BrokenProcessPool`` exception instance among all failed futures. This was unsafe because exceptions are mutable. Malformed (repeated) tracebacks were the most common symptom of this bug.
+When a worker process terminates unexpectedly,
+:class:`concurrent.futures.ProcessPoolExecutor` now sets a separate
+:exc:`~concurrent.futures.process.BrokenProcessPool` exception on each pending
+future instead of sharing a single instance among them all.  Sharing one
+exception produced malformed tracebacks: each
+:meth:`Future.result() <concurrent.futures.Future.result>` call re-raised the
+same object, appending another copy of the traceback to it.