Download: Polish a comment for transient flag in DownloadUrlParameters.

This CL polishes the comment for transient flag in DownloadUrlParameters.

Change-Id: I06732268dc08889cc195b61d3140560acc9e5242
Reviewed-on: https://chromium-review.googlesource.com/c/1357380Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612765}

components/download/public/common/download_url_parameters.h

@@ -215,9 +215,10 @@ class COMPONENTS_DOWNLOAD_EXPORT DownloadUrlParameters {
     fetch_error_body_ = fetch_error_body;
   }
 
-  // Sets whether the download is to be treated as transient. A transient
-  // download is short-lived and is not shown in the UI, and will not prompt
-  // to user for target file path determination.
+  // A transient download will not be shown in the UI, and will not prompt
+  // to user for target file path determination. Transient download should be
+  // cleared properly through DownloadManager to avoid the database and
+  // in-memory DownloadItem objects accumulated for the user.
   void set_transient(bool transient) { transient_ = transient; }
 
   // Sets the optional guid for the download, the guid serves as the unique