Update descriptions of request_id

Reinforce API descriptions to make it easier to use

Change-Id: I436bc5aa8e70a7a8aa5ffeab968a3ba3983bb371
Signed-off-by: Minje Ahn <minje.ahn@samsung.com>

diff --git a/include/thumbnail_util.h b/include/thumbnail_util.h
index 383072a..0482905 100755 (executable)
--- a/include/thumbnail_util.h
+++ b/include/thumbnail_util.h
@@ -55,6 +55,8 @@ int thumbnail_util_create(thumbnail_h *thumb);
  * @brief Extracts the thumbnail for the given media, asynchronously.
  * @details This function extracts the thumbnail for given media item and calls registered
  *         callback function for completion of extracting the thumbnail.\n
+ *         You can distinguish generated thumbnails by @a request_id. Therefore, the @a request_id must be maintained until the request is completed. \n
+ *         And the @a request_id is also used to cancel the request.
  *
  * @since_tizen @if MOBILE 2.4 @elseif WEARABLE 3.0 @endif
  *