message: Document the configurable-env usntable option
diff: @@ -1130,3 +1130,29 @@ The 2021 edition will set the default [resolver version] to "2".
  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.
  +
  +### configurable-env
  +* Original Pull Request: [#9175](https://github.com/rust-lang/cargo/pull/9175)
  +
  +The `-Z configurable-env` flag enables the `[env]` section in the
  +`.cargo/config.toml` file. This section allows you to set additional environment
  +variables for build scripts, rustc invocations, `cargo run` and `cargo build`.
  +
  +```toml
  +[env]
  +OPENSSL_DIR = "/opt/openssl"
  +```
  +
  +By default, the variables specified will not override values that already exist
  +in the environment. This behavior can be changed by setting the `force` flag.
  +
  +Setting the `relative` flag evaluates the value as a config-relative path that
  +is relative to the parent directory of the `.cargo` directory that contains the
  +`config.toml` file. The value of the environment variable will be the full
  +absolute path.
  +
  +```toml
  +[env]
  +TMPDIR = { value = "/home/tmp", force = true }
  +OPENSSL_DIR = { value = "vendor/openssl", relative = true }
  +```
  