add comment explaining issue with latest version

Author: alexbruy

mergin/client_pull.py

@@ -618,6 +618,8 @@ def download_file_async(mc, project_dir, file_path, output_file, version):
     download_list = []
     update_tasks = []
     total_size = 0
+    # None can not be used to indicate latest version of the file, so
+    # it is necessary to pass actual version.
     if version is None:
         version = latest_proj_info["version"]
     for file in project_info['files']: