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>

drivers/staging/media/ipu3/include/uapi/intel-ipu3.h

@@ -234,7 +234,9 @@ struct ipu3_uapi_ae_ccm {
  * struct ipu3_uapi_ae_config - AE config
  *
  * @grid_cfg:	config for auto exposure statistics grid. See struct
- *		&ipu3_uapi_ae_grid_config
+ *		&ipu3_uapi_ae_grid_config, as Imgu did not support output
+ *		auto exposure statistics, so user can ignore this configuration
+ *		and use the RGB table in auto-whitebalance statistics instead.
  * @weights:	&IPU3_UAPI_AE_WEIGHTS is based on 32x24 blocks in the grid.
  *		Each grid cell has a corresponding value in weights LUT called
  *		grid value, global histogram is updated based on grid value and
@@ -534,6 +536,9 @@ struct ipu3_uapi_ff_status {
  *
  * @awb_raw_buffer: auto white balance meta data &ipu3_uapi_awb_raw_buffer
  * @ae_raw_buffer: auto exposure raw data &ipu3_uapi_ae_raw_buffer_aligned
+ *                 current Imgu does not output the auto exposure statistics
+ *                 to ae_raw_buffer, the user such as 3A algorithm can use the
+ *                 RGB table in &ipu3_uapi_awb_raw_buffer to do auto-exposure.
  * @af_raw_buffer: &ipu3_uapi_af_raw_buffer for auto focus meta data
  * @awb_fr_raw_buffer: value as specified by &ipu3_uapi_awb_fr_raw_buffer
  * @stats_4a_config: 4a statistics config as defined by &ipu3_uapi_4a_config.