Added the new lint with some docs

Author: botahamec

CHANGELOG.md

@@ -4648,6 +4648,7 @@ Released 2018-09-13
 [`unstable_as_mut_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_mut_slice
 [`unstable_as_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_slice
 [`unused_async`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_async
+[`unused_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_box
 [`unused_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_collect
 [`unused_format_specs`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_format_specs
 [`unused_io_amount`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount