Ie, state that "use" statements can start with an identifier, resolved in a
similar way to a path expression.

Now the main text and example in use-declarations.md describe the 2018 edition
semantics, and the "Edition Differences" section says how the 2015 edition
differs.

Now the main text describes what is true in both 2015 and 2018 editions; the
varying behaviour (and all examples) are in an "Edition differences" section.

For the behaviour which doesn't vary between editions, avoids repeating
resolution rules which are documented elsewhere.