doc: release-notes-4-2: Add coredump new Kconfig options

A new coredump backend was added, and can be enabled and configured
directly via Kconfig.

Signed-off-by: Tomasz Bursztyka <tobu@bang-olufsen.dk>

Author: tbursztyka

doc/releases/release-notes-4.2.rst

@@ -232,6 +232,8 @@ New APIs and options
   * Core Dump
 
     * :kconfig:option:`CONFIG_DEBUG_COREDUMP_THREAD_STACK_TOP`, enabled by default for ARM Cortex M when :kconfig:option:`CONFIG_DEBUG_COREDUMP_MEMORY_DUMP_MIN` is selected.
+    * :kconfig:option:`CONFIG_DEBUG_COREDUMP_BACKEND_IN_MEMORY`
+    * :kconfig:option:`CONFIG_DEBUG_COREDUMP_BACKEND_IN_MEMORY_SIZE`
 
 * Other