doc: releases: add shell mqtt backend kconfigs

Add shell MQTT backend Kconfigs to release notes from commits
ddc4479 and
7c796c4.

Signed-off-by: Jeppe Odgaard <[email protected]>

Author: Jeppe Odgaard

doc/releases/release-notes-4.3.rst

@@ -172,6 +172,16 @@ New APIs and options
 
    * :kconfig:option:`CONFIG_SETTINGS_TFM_ITS`
 
+* Shell
+
+   * MQTT backend
+
+      * :kconfig:option:`CONFIG_SHELL_MQTT_TOPIC_RX_ID`
+      * :kconfig:option:`CONFIG_SHELL_MQTT_TOPIC_TX_ID`
+      * :kconfig:option:`CONFIG_SHELL_MQTT_CONNECT_TIMEOUT_MS`
+      * :kconfig:option:`CONFIG_SHELL_MQTT_WORK_DELAY_MS`
+      * :kconfig:option:`CONFIG_SHELL_MQTT_LISTEN_TIMEOUT_MS`
+
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