boards: arm: mimxrt1040_evk: document M.2 WiFi module debugging issue

Document issue with M.2 WiFi module, where debug connection fails using
external debugger. Current workaround is to remove J80 jumper when using
onboard debugger, or remove M.2 module entirely.

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