doc: releases: Add runtime changes to release notes

Add runtime observers static allocation feature to release notes.

Signed-off-by: Rodrigo Peixoto <[email protected]>

Author: rodrigopex

doc/releases/release-notes-4.2.rst

@@ -179,6 +179,13 @@ New APIs and options
 * LoRaWAN
    * :c:func:`lorawan_request_link_check`
 
+* Zbus
+  * Runtime observers can work without heap. Now it is possible to choose between static and dynamic
+    allocation for the runtime observers nodes.
+  * :kconfig:option:`CONFIG_ZBUS_RUNTIME_OBSERVERS_NODE_ALLOC_DYNAMIC`
+  * :kconfig:option:`CONFIG_ZBUS_RUNTIME_OBSERVERS_NODE_ALLOC_STATIC`
+  * :kconfig:option:`CONFIG_ZBUS_RUNTIME_OBSERVERS_NODE_POOL_SIZE`
+
 New Boards
 **********