modem: modem_cellular: Migration guide for CMUX

Add migration guide for configuring MTU for CMUX.

Signed-off-by: Seppo Takalo <[email protected]>

Author: SeppoTakalo

doc/releases/migration-guide-4.2.rst

@@ -154,6 +154,13 @@ Timer
   :kconfig:option:`CONFIG_NATIVE_POSIX_TIMER` has been deprecated in favor of
   :kconfig:option:`CONFIG_NATIVE_SIM_TIMER`, (:github:`86612`).
 
+Modem
+=====
+
+* Removed Kconfig option :kconfig:option:`CONFIG_MODEM_CELLULAR_CMUX_MAX_FRAME_SIZE` in favor of
+  :kconfig:option:`CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE` and :kconfig:option:`CONFIG_MODEM_CMUX_MTU`.
+
+
 Bluetooth
 *********