[document_repository] add usage doc for filter (#7910)

Explain how the filter works in the document_repository help section.

Resolves #7865

modules/document_repository/help/document_repository.md

@@ -3,7 +3,8 @@
 This module provides studies with a centralized location for important documents. You can organize files by categories and subcategories designed by you or other members of your team.
 
 In the **Browse** tab, you can search for specific files using the **Selection Filter** section. Click on any file name in the data table to download. Click on any of the folders to reveal subfolders and files organized within. You can click on **Edit** or **Delete** in any cell, in those respective columns, to modify a file. Click on certain column headers to sort the list of documents.
-
+The **Filter globally** only works in the current directory (root folder only contains directories).
+The **Filter globally** will filter the file across the directories.
 Navigate to the **Upload** tab to upload a file. Populate the fields and upload your file. Your newly uploaded file should now appear in the list within the Browse tab.
 
 If you need to add a new category or subcategory, you can do so in the **Category** tab. Populate the fields and click **Add Category**.