Add documentation for -Zcheck-target-cfgs

Author: Urgau

src/doc/src/reference/unstable.md

@@ -118,6 +118,7 @@ Each new feature described below should explain how to use it.
     * [lockfile-path](#lockfile-path) --- Allows to specify a path to lockfile other than the default path `<workspace_root>/Cargo.lock`.
     * [package-workspace](#package-workspace) --- Allows for packaging and publishing multiple crates in a workspace.
     * [native-completions](#native-completions) --- Move cargo shell completions to native completions.
+    * [check-target-cfgs](#check-target-cfgs) --- Allows checking unexpected cfgs in `[target.'cfg(...)']`
 
 ## allow-features
 
@@ -1728,6 +1729,29 @@ When in doubt, you can discuss this in [#14520](https://github.com/rust-lang/car
 - powershell:
   Add `CARGO_COMPLETE=powershell cargo +nightly | Invoke-Expression` to `$PROFILE`.
 
+## check-target-cfgs
+
+* Tracking Issue: [#00000](https://github.com/rust-lang/cargo/issues/00000)
+
+This feature checks for unexpected cfgs in `[target.'cfg(...)']` entries, based
+on `rustc --print=check-cfg`. It does **not** follow `RUSTFLAGS` but instead use
+the lint configuration.
+
+```sh
+cargo check -Zcheck-target-cfgs
+```
+
+It follows the lint Rust `unexpected_cfgs` lint configuration:
+
+```toml
+[target.'cfg(foo)'.dependencies]
+cfg-if = "1.0"
+
+[lints.rust.unexpected_cfgs]
+level = "warn"
+check-cfg = ['cfg(foo)']
+```
+
 # Stabilized and removed features
 
 ## Compile progress