message: Document per-target edition & give an example
diff: @@ -187,7 +187,10 @@ cargo +nightly build --out-dir=out -Z unstable-options
  You can opt in to a specific Rust Edition for your package with the `edition`
  key in `Cargo.toml`. If you don't specify the edition, it will default to
  -2015. You need to include the appropriate `cargo-features`:
  +2015. You need to include the appropriate `cargo-features`.
  +
  +You can also specify `edition` on a per-target level, where it will otherwise
  +default to the package `edition`.
  ```toml
  cargo-features = ["edition"]
  @@ -195,6 +198,10 @@ cargo-features = ["edition"]
  [package]
  ...
  edition = "2018"
  +
  +[[bin]]
  +...
  +edition = "2015"
  ```
  