Make CHANGES.md header more appropriate

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from #22785)

CHANGES.md

@@ -1,9 +1,11 @@
 OpenSSL CHANGES
 ===============
 
-This is a high-level summary of the most important changes.
-For a full list of changes, see the [git commit log][log] and
-pick the appropriate release branch.
+This is a detailed breakdown of significant changes. For a high-level overview
+of changes in each release, see [NEWS.md](./NEWS.md).
+
+For a full list of changes, see the [git commit log][log] and pick the
+appropriate release branch.
 
   [log]: https://github.com/openssl/openssl/commits/