doc: release: 3.3: Add os_mgmt info command

Adds details of a new os_mgmt command for getting OS/application
information.

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

Author: nordicjm

doc/releases/release-notes-3.3.rst

@@ -400,6 +400,9 @@ Libraries / Subsystems
 
     Private headers for above areas can be accessed, when required, using paths:
     ``mgmt/mcumgr/mgmt/<mcumgr_subarea>/``.
+  * MCUmgr os_mgmt info command has been added that allows querying details on
+    the kernel and application, allowing application-level extensibility
+    see :ref:`mcumgr_os_application_info` for details.
 
 * LwM2M