CameraNDK: add NDK API documents

This change adds document for ACameraManager, ACameraDevice and
ACameraCaptureSession.

Bug: 27102995

Change-Id: I5706337e419cd533c1c7d46e412dc62bf7c726cb
diff --git a/include/camera/ndk/NdkCameraMetadata.h b/include/camera/ndk/NdkCameraMetadata.h
index 9b56a9d..8a8865d 100644
--- a/include/camera/ndk/NdkCameraMetadata.h
+++ b/include/camera/ndk/NdkCameraMetadata.h
@@ -14,6 +14,15 @@
  * limitations under the License.
  */
 
+/**
+ * @addtogroup Camera
+ * @{
+ */
+
+/**
+ * @file NdkCameraMetadata.h
+ */
+
 /*
  * This file defines an NDK API.
  * Do not remove methods.
@@ -119,3 +128,5 @@
 #endif
 
 #endif //_NDK_CAMERA_METADATA_H
+
+/** @} */