doc: releases: migration-guide-4.0: document change to USDHC card detect

NXP IMX USDHC fallback card detection method changed from querying the
peripheral to simply assuming the card was present. Add a note to the
migration guide highlighting this and explaining how to restore the
previous behavior for anyone using this detection method.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>

Author: danieldegrasse

doc/releases/migration-guide-4.0.rst

@@ -88,6 +88,14 @@ Interrupt Controller
 LED Strip
 =========
 
+SDHC
+====
+
+* The NXP USDHC driver now assumes a card is present if no card detect method
+  is configured, instead of using the peripheral's internal card detect signal
+  to check for card presence. To use the internal card detect signal, the
+  devicetree property ``detect-cd`` should be added to the USDHC node in use.
+
 Sensors
 =======