update docs about sfs/avatar.index

dump-html.py

@@ -17,7 +17,7 @@ def get_args():
     parser.add_argument('name', help='name of contact')
     parser.add_argument('--output', help='output html file', default='output.html')
     parser.add_argument('--db', default='decoded.db', help='path to decoded database')
-    parser.add_argument('--avt', default='avatar.index', help='path to avatar.index file')
+    parser.add_argument('--avt', default='avatar.index', help='path to avatar.index file that only exists in old version of wechat')
     parser.add_argument('--res', default='resource', help='reseource directory')
     args = parser.parse_args()
     return args

wechat/res.py

@@ -30,6 +30,11 @@ JPEG_QUALITY = 50
 class Resource(object):
     """ multimedia resources in chat"""
     def __init__(self, parser, res_dir, avt_db):
+        """
+        Args:
+            res_dir: path to the resource directory
+            avt_db: "avatar.index" file that only exists in old versions of wechat
+        """
         def check(subdir):
             dir_to_check = os.path.join(res_dir, subdir)
             assert os.path.isdir(dir_to_check), f"No such directory: {dir_to_check}"