doc: migration: 4.4: add section about CONFIG_NUM_IRQS on STM32

As CONFIG_NUM_IRQS is now determined dynamically, it is possible that apps
which registered ISRs outside drivers will trigger a build failure if the
registered IRQn is higher than the biggest active device's IRQn.

Add a note in the migration guide which describes this problem and gives
the solution to tackle these build failures.

Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>

Author: mathieuchopstm

doc/releases/migration-guide-4.4.rst

@@ -947,6 +947,17 @@ STM32
   the ``cs-gpios`` or new ``st,soft-nss`` property operate in "Soft NSS" mode, while all other
   instances operate in "Hard NSS" mode.
 
+* :kconfig:option:`CONFIG_NUM_IRQS` is computed automatically based on active (``status = "okay";``)
+  devices by using the new ``dt_highest_controller_irq_number`` Kconfig preprocessor function.
+  Applications which register custom ISRs (using :c:macro:`IRQ_CONNECT()`) may encounter build
+  failures such as the following due to :kconfig:option:`CONFIG_NUM_IRQS` having a lower value:
+
+  .. code-block::
+
+    gen_isr_tables.py: error: IRQ 114 (offset=0) exceeds the maximum of 106
+
+  Explicitly set :kconfig:option:`CONFIG_NUM_IRQS` to an appropriate value to solve these issues.
+
 Timer
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