doc: release: 3.5: Add additional retention change note

Adds a note on the change to disable retention system mutex
support

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

Author: nordicjm

doc/releases/release-notes-3.5.rst

@@ -397,6 +397,9 @@ Libraries / Subsystems
 
   * Added the :ref:`blinfo_api` subsystem.
 
+  * Added support for allowing mutex support to be forcibly disabled with
+    :kconfig:option:`CONFIG_RETENTION_MUTEX_FORCE_DISABLE`.
+
 * Binary descriptors
 
   * Added the :ref:`binary_descriptors` (``bindesc``) subsystem.