graphics: Update RAW16 format comments

     *    - GRALLOC_USAGE_SW_*

     *    - GRALLOC_USAGE_RENDERSCRIPT

     *

     * When used with ANativeWindow, the dataSpace should be

     * HAL_DATASPACE_ARBITRARY, as raw image sensor buffers require substantial

     * extra metadata to define.

     * When used with ANativeWindow, the mapping of the dataSpace field to

     * buffer contents for RAW16 is as follows:

     *

     *  dataSpace value               | Buffer contents

     * -------------------------------+-----------------------------------------

     *  HAL_DATASPACE_ARBITRARY       | Raw image sensor data, layout is as

     *                                | defined above.

     *  HAL_DATASPACE_DEPTH           | Unprocessed implementation-dependent raw

     *                                | depth measurements, opaque with 16 bit

     *                                | samples.

     *  Other                         | Unsupported

     */

    RAW16 = 0x20,