message: Add doc for -Zextra-check-cfg and rustc-check-cfg
diff: @@ -1201,6 +1201,7 @@ It's values are:
  Note than this command line options will probably become the default when stabilizing.
  - `names`: enables well known names checking via `--check-cfg=names()`.
  - `values`: enables well known values checking via `--check-cfg=values()`.
  + - `output`: enable the use of `rustc-check-cfg` in build script.
  For instance:
  @@ -1211,6 +1212,29 @@ cargo check -Z unstable-options -Z check-cfg=values
  cargo check -Z unstable-options -Z check-cfg=features,names,values
  ```
  +Or for `output`:
  +
  +```rust,no_run
  +// build.rs
  +println!("cargo:rustc-check-cfg=names(foo, bar)");
  +```
  +
  +```
  +cargo check -Z unstable-options -Z check-cfg=output
  +```
  +
  +### `cargo:rustc-check-cfg=CHECK_CFG`
  +
  +The `rustc-check-cfg` instruction tells Cargo to pass the given value to the
  +`--check-cfg` flag to the compiler. This may be used for compile-time
  +detection of unexpected conditional compilation name and/or values.
  +
  +This can only be used in combination with `-Zcheck-cfg=output` otherwise it is ignored
  +with a warning.
  +
  +If you want to integrate with Cargo features, use `-Zcheck-cfg=features` instead of
  +trying to do it manually with this option.
  +
  ### workspace-inheritance
  * RFC: [#2906](https://github.com/rust-lang/rfcs/blob/master/text/2906-cargo-workspace-deduplicate.md)
  