Add docstring

Lib/cmd.py

@@ -169,6 +169,10 @@ class Cmd:
                 pass
 
     def parseline(self, line):
+        """Parse the line into a command name and a string containing
+        the arguments.  Returns a tuple containing (command, args, line).
+        'command' and 'args' may be None if the line couldn't be parsed.
+        """
         line = line.strip()
         if not line:
             return None, None, line