docs: release-notes: Add snippet on encrypted scratch area

Add release note snippet regarding the change made to the swap with
scratch algorithm to avoid having plaintext firmware data stored in the
scratch area.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>

docs/release-notes.d/encrypted-scratch-partition.md

@@ -0,0 +1,3 @@
+- When using swap with scratch, the image is now decrypted when copying from
+  the scratch partition to the primary slot. Therefore, the sratch partition
+  doesn't contain plaintext firmware data anymore.