Document ability to list subdirectories.

Author: Daniel Dunn

docs/Usage/Storage Access Framework.md

@@ -71,7 +71,7 @@ This method list files lazily **over a granted uri:**
 > **Note** `DocumentFileColumn.id` is optional. It is required to fetch the file list from native API. So it is enabled regardless if you include this column or not. And this applies only to this API (`listFiles`).
 
 ```dart
-/// *Must* be a granted uri from `openDocumentTree`
+/// *Must* be a granted uri from `openDocumentTree`, or a URI representing a child under such a granted uri.
 final Uri myGrantedUri = ...
 final DocumentFile? documentFileOfMyGrantedUri = await myGrantedUri.toDocumentFile();
 
@@ -598,6 +598,7 @@ This class represents but is not the mirror of the original [`DocumentFile`](htt
 
 This class is not intended to be instantiated, and it is only used for typing and convenient purposes.
 
+
 ### <samp>QueryMetadata</samp>
 
 This class wraps useful metadata of the source queries returned by the `PartialDocumentFile`.