Camera: Clarify AF_REGIONS behavior in capture_result

If application specifies 0-weight AF_REGIONS in capture request, the
camera device is allowed to override with non-0-weight AF_REGIONS in
capture result.

Test: Build
Bug: 29398609
Change-Id: I1e90c6c1717a894458b9a989cbd25981eaad43fb

camera/ndk/include/camera/NdkCameraMetadataTags.h

@@ -651,7 +651,8 @@ typedef enum acamera_metadata_tag {
      * is used, all non-zero weights will have the same effect. A region with 0 weight is
      * ignored.</p>
      * <p>If all regions have 0 weight, then no specific metering area needs to be used by the
-     * camera device.</p>
+     * camera device. The capture result will either be a zero weight region as well, or
+     * the region selected by the camera device as the focus area of interest.</p>
      * <p>If the metering region is outside the used ACAMERA_SCALER_CROP_REGION returned in
      * capture result metadata, the camera device will ignore the sections outside the crop
      * region and output only the intersection rectangle as the metering region in the result