easy-menu-define docstring inconsistent with behavior

[James Clark]

The docstring for easy-menu-define says:

> The first element of MENU must be a string.  It is the menu bar item name.
> It may be followed by the following keyword argument pairs
> 
>    :filter FUNCTION
> 
> FUNCTION is a function with one argument, the menu.  It returns the actual
> menu displayed.

In fact, the argument and return value for function are the remaining
menu items not the entire menu.

James