Move the documentation for maybe_decode into a docstring on the method itself.

diffoscope/comparators/utils/file.py

@@ -404,10 +404,12 @@ class File(object, metaclass=abc.ABCMeta):
             return difference
         return self.compare_bytes(other, source)
 
-# helper function to convert to bytes if necessary
-
 
 def maybe_decode(s):
+    """
+    Helper function to convert to bytes if necessary.
+    """
+
     if type(s) is bytes:
         return s.decode('utf-8')
     return s