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>