doc/releases: Add not on mcumgr support for non-0xff erase flash

The commit adds information, to release notes, on mcumgr now supporting
flash devices that have non-0xff erase value.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/doc/releases/release-notes-2.5.rst b/doc/releases/release-notes-2.5.rst
index 984da69..f64fe8f 100644
--- a/doc/releases/release-notes-2.5.rst
+++ b/doc/releases/release-notes-2.5.rst
@@ -245,6 +245,8 @@
 
   * MCUmgr
 
+    * Added support for flash devices that have non-0xff erase value.
+
   * updatehub
 
 * Settings