doc: release: 4.1: Add MCUmgr callback change

Adds a note of the new data in the callback

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

doc/releases/release-notes-4.1.rst

@@ -138,6 +138,13 @@ New APIs and options
   * :kconfig:option:`CONFIG_MBEDTLS_PSA_STATIC_KEY_SLOTS`
   * :kconfig:option:`CONFIG_MBEDTLS_PSA_KEY_SLOT_COUNT`
 
+* Management
+
+  * MCUmgr
+
+    * Image management :c:macro:`MGMT_EVT_OP_IMG_MGMT_DFU_CONFIRMED` now has image data field
+      :c:struct:`img_mgmt_image_confirmed`.
+
 * Other
 
   * :kconfig:option:`CONFIG_BT_BUF_ACL_RX_COUNT_EXTRA`