This adds strictly more information to the source files and reduces the
need for customized tooling to render the book.
(While this should not change the output of _rustbook_, it is very
useful when rendering the sources with external tools like Pandoc.)
If you have 0 references (`&T`) to a resource, presumably, you could
have a mutable reference (`&mut T`). So this only start to make sense
at having 1 reference to a resource.
