(#50) Warn that DocumentFileColumn.id is optional on listFiles docs

Author: LaksCastro

docs/Usage/Storage Access Framework.md

@@ -68,6 +68,8 @@ if (grantedUri != null) {
 
 This method list files lazily **over a granted uri:**
 
+> **Info** `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`
 final Uri myGrantedUri = ...
@@ -83,7 +85,7 @@ const List<DocumentFileColumn> columns = <DocumentFileColumn>[
   DocumentFileColumn.displayName,
   DocumentFileColumn.size,
   DocumentFileColumn.lastModified,
-  DocumentFileColumn.id,
+  DocumentFileColumn.id, // Optional column, will be available/queried regardless if is or not included here
   DocumentFileColumn.mimeType,
 ];