bug#11327: 24.1.50; doc of `dired-do-create-files' refers user to undocumented var/fn

[Chong Yidong]

> The doc string says:
> ... "The way the default for the target directory is computed depends on
> the value of `dired-dwim-target-directory'."
> There is no variable with that name.  And a function does not have a
> "value".

The "value" of a function is its return value.
See (elisp)What Is a Function

But anyway, I reworded the docstring to make it clearer.