doc: release: 4.2: Add MCUmgr changes

Adds new features/APIs for MCUmgr

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

Author: nordicjm

doc/releases/release-notes-4.2.rst

@@ -242,6 +242,15 @@ New APIs and options
 
   * :c:func:`display_clear`
 
+* Management
+
+  * MCUmgr
+
+    * Firmware loader support added to image mgmt group using
+      :kconfig:option:`CONFIG_MCUBOOT_BOOTLOADER_MODE_FIRMWARE_UPDATER`.
+    * Optional boot mode (using retention boot mode) added to OS group reset command using
+      :kconfig:option:`CONFIG_MCUMGR_GRP_OS_RESET_BOOT_MODE`.
+
 * Networking:
 
   * IPv4