[clang-tidy] Add cppcoreguidelines-prefer-member-initializer to ReleaseNotes

[njames93]

Summary

Following a discussion about the current state of this check on the 12.X branch, it was decided to purge the check as it wasn't in a fit to release state, see https://llvm.org/PR49318.
This check has since had some of those issues addressed and should be good for the next release cycle now, pending any more bug reports about it.

[aaron.ballman]

LGTM, thank you!

clang-tools-extra/docs/ReleaseNotes.rst

 ^^^^^^^^^^
 
 - New :doc:`concurrency-thread-canceltype-asynchronous
   <clang-tidy/checks/concurrency-thread-canceltype-asynchronous>` check.
 
   Finds ``pthread_setcanceltype`` function calls where a thread's cancellation
   type is set to asynchronous.
 
+- New :doc:`cppcoreguidelines-prefer-member-initializer
+  <clang-tidy/checks/cppcoreguidelines-prefer-member-initializer>` check.
+
+  Finds member initializations in the constructor body which can be placed into
+  the initialization list instead.
+
 New check aliases
 ^^^^^^^^^^^^^^^^^
 
 - New alias :doc:`cert-pos47-c
   <clang-tidy/checks/cert-pos47-c>` to
   :doc:`concurrency-thread-canceltype-asynchronous
   <clang-tidy/checks/concurrency-thread-canceltype-asynchronous>` was added.