changelog: enhance description

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/ChangeLog.d/8709.txt b/ChangeLog.d/8709.txt
index 52b9609..a95cedf 100644
--- a/ChangeLog.d/8709.txt
+++ b/ChangeLog.d/8709.txt
@@ -1,3 +1,5 @@
 Features
    * The new function mbedtls_pk_copy_from_psa() provides a way to setup
-     a PK context starting from a PSA key.
+     a PK context starting from a PSA key. In contrast to
+     mbedtls_pk_setup_opaque() the resulting PK context is a normal context
+     that's independent from the original PSA key.