Doc: clarify ordering rule for default arguments in tutorial

Author: jeff14994

Doc/tutorial/controlflow.rst

@@ -588,8 +588,9 @@ Default Argument Values
 -----------------------
 
 The most useful form is to specify a default value for one or more arguments.
-This creates a function that can be called with fewer arguments than it is
-defined to allow.  For example::
+All arguments without default values must come first, followed by those with
+defaults. This allows a function to be called with fewer arguments than it is
+defined to allow. For example::
 
    def ask_ok(prompt, retries=4, reminder='Please try again!'):
        while True: