doc: release-notes-4.3: Add entry about CoAP client payload callback

Add release notes entry covering the new CoAP client payload callback.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

doc/releases/release-notes-4.3.rst

@@ -212,6 +212,7 @@ New APIs and options
   * CoAP
 
     * :c:struct:`coap_client_response_data`
+    * :c:member:`coap_client_request.payload_cb`
 
   * Sockets