I just ran across the other reason that having the actual default values documented is important. Sometimes I want to do this:

some_func(param if some_condition else <use the default value>)

If some_condition is False, I want the default behavior, if not, I want to pass in a parameter. If I don't know the real default value, I have to write:

if some_condition:
   some_func(param)
else:
   some_func()