doc: release-notes: Add mcumgr file read/write request hook details

Adds the file read/write request hook that applications can use to allow
or decline requests to files to the release notes.

Signed-off-by: Jamie McCrae <[email protected]>

Author: Jamie McCrae

doc/releases/release-notes-3.1.rst

@@ -246,6 +246,9 @@ Libraries / Subsystems
 
   * Added mcumgr os hook to allow an application to accept or decline a reset
     request; :kconfig:option:`CONFIG_OS_MGMT_RESET_HOOK` enables the callback.
+  * Added mcumgr fs hook to allow an application to accept or decline a file
+    read/write request; :kconfig:option:`CONFIG_FS_MGMT_FILE_ACCESS_HOOK`
+    enables the feature which then needs to be registered by the application.
 
 * SD Subsystem