media: Documentation: Add ST VGXY61 driver documentation

Document V4L2_CID_HDR_MODE possible values for this sensor.
Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>

Documentation/userspace-api/media/drivers/st-vgxy61.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+ST VGXY61 camera sensor driver
+==============================
+
+The ST VGXY61 driver implements the following controls:
+
+``V4L2_CID_HDR_SENSOR_MODE``
+-------------------------------
+    Change the sensor HDR mode. A HDR picture is obtained by merging two captures of the same scene
+    using two different exposure periods.
+
+.. flat-table::
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       1 4
+
+    * - HDR linearize
+      - The merger outputs a long exposure capture as long as it is not saturated.
+    * - HDR substraction
+      - This involves subtracting the short exposure frame from the long exposure frame.
+    * - "No HDR"
+      - This mode is used for standard dynamic range (SDR) exposures.