doc: releases: Add a kernel release entry

Add the `k_thread_runtime_stats_is_enabled` function
to check if runtime statistics are enabled.

Signed-off-by: James Roy <rruuaanng@outlook.com>

Author: rruuaanng

doc/releases/release-notes-4.2.rst

@@ -314,6 +314,12 @@ New APIs and options
   * :kconfig:option:`CONFIG_ZBUS_RUNTIME_OBSERVERS_NODE_POOL_SIZE`
 
 
+Kernel
+******
+
+* :c:func:`k_thread_runtime_stats_is_enabled`.
+
+
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