Update multiple file use statement example

Update the example to make the usage of `pub mod foo;` much more
apparent, as well as using an example where setting the visibility of
the module is actually necessary.
auto merge of #13960 : brandonw/rust/master, r=alexcrichton

Update the example to make the usage of `pub mod foo;` much more
apparent, as well as using an example where setting the visibility of
the module is actually necessary.