media: staging: document that Imgu not output auto-exposure statistics

Currently, Imgu can not support output the auto-exposure statistics into
ae_raw_buffer in 3A stats buffer, this patch document it.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>