With the recent addition of a generic device_detect for MCI devices,
the MCI_STARTUP help text is no longer accurate. Reword it, but leave
the option as-is as some boards may be broken if we drop the option
(e.g. because they still depend on a probe order or call
default_environment_path_set, but don't explicitly detect the device).

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>