message: Elaborate a little on future-incompat unstable docs.
diff: @@ -1150,9 +1150,18 @@ The 2021 edition will set the default [resolver version] to "2".
  * RFC: [#2834](https://github.com/rust-lang/rfcs/blob/master/text/2834-cargo-report-future-incompat.md)
  * rustc Tracking Issue: [#71249](https://github.com/rust-lang/rust/issues/71249)
  -The `-Z future-incompat-report` flag enables the creation of a future-incompat report
  -for all dependencies. This makes users aware if any of their crate's dependencies
  -might stop compiling with a future version of Rust.
  +The `-Z future-incompat-report` flag causes Cargo to check for
  +future-incompatible warnings in all dependencies. These are warnings for
  +changes that may become hard errors in the future, causing the dependency to
  +stop building in a future version of rustc. If any warnings are found, a small
  +notice is displayed indicating that the warnings were found, and provides
  +instructions on how to display a full report.
  +
  +A full report can be displayed with the `cargo report future-incompatibilities
  +-Z future-incompat-report --id ID` command, or by running the build again with
  +the `--future-incompat-report` flag. The developer should then update their
  +dependencies to a version where the issue is fixed, or work with the
  +developers of the dependencies to help resolve the issue.
  ### configurable-env
  * Original Pull Request: [#9175](https://github.com/rust-lang/cargo/pull/9175)
  