Adjust documentation for newly stabilized -Zcheck-cfg

src/doc/src/reference/build-scripts.md

@@ -130,6 +130,8 @@ one detailed below.
   compiler.
 * [`cargo::rustc-cfg=KEY[="VALUE"]`](#rustc-cfg) --- Enables compile-time `cfg`
   settings.
+* [`cargo::rustc-check-cfg=CHECK_CFG`](#rustc-check-cfg) -- Enables compile-time
+  checking of configs. 
 * [`cargo::rustc-env=VAR=VALUE`](#rustc-env) --- Sets an environment variable.
 * [`cargo::rustc-cdylib-link-arg=FLAG`](#rustc-cdylib-link-arg) --- Passes custom
   flags to a linker for cdylib crates.
@@ -233,7 +235,9 @@ equivalent to using [`rustc-link-lib`](#rustc-link-lib) and
 
 The `rustc-cfg` instruction tells Cargo to pass the given value to the
 [`--cfg` flag][option-cfg] to the compiler. This may be used for compile-time
-detection of features to enable [conditional compilation].
+detection of features to enable [conditional compilation]. Each custom cfgs
+must **always** be declared using the [`cargo::rustc-check-cfg`](#rustc-check-cfg)
+instruction.
 
 Note that this does *not* affect Cargo's dependency resolution. This cannot be
 used to enable an optional dependency, or enable other Cargo features.
@@ -250,6 +254,25 @@ identifier, the value should be a string.
 [conditional compilation]: ../../reference/conditional-compilation.md
 [option-cfg]: ../../rustc/command-line-arguments.md#option-cfg
 
+### `cargo::rustc-check-cfg=CHECK_CFG` {#rustc-check-cfg}
+
+The `rustc-check-cfg` instruction tells Cargo to pass the given expected
+configs to the [`--check-cfg` flag][option-check-cfg] to the compiler. This is
+used for compile-time detection of unexpected conditional compilation configs.
+
+You can use the instruction like this:
+
+```rust,no_run
+// build.rs
+println!("cargo::rustc-check-cfg=cfg(foo)");
+```
+
+Note that it is expected to **always** pass the full list of custom configs
+used in the package. The recommended way to achieve this is put all the
+check-cfg instructions at the top of your `build.rs`.
+
+[option-cfg]: ../../rustc/command-line-arguments.md#option-check-cfg
+
 ### `cargo::rustc-env=VAR=VALUE` {#rustc-env}
 
 The `rustc-env` instruction tells Cargo to set the given environment variable

src/doc/src/reference/unstable.md

@@ -1126,44 +1126,6 @@ You can use the flag like this:
 cargo rustdoc -Z unstable-options --output-format json
 ```
 
-## check-cfg
-
-* RFC: [#3013](https://github.com/rust-lang/rfcs/pull/3013)
-* Tracking Issue: [#10554](https://github.com/rust-lang/cargo/issues/10554)
-
-`-Z check-cfg` command line enables compile time checking of Cargo features as well as `rustc`
-well known names and values in `#[cfg]`, `cfg!`, `#[link]` and `#[cfg_attr]` with the `rustc`
-and `rustdoc` unstable `--check-cfg` command line.
-
-You can use the flag like this:
-
-```
-cargo check -Z unstable-options -Z check-cfg
-```
-
-### `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` otherwise it is ignored
-with a warning.
-
-If you want to integrate with Cargo features, only use `-Zcheck-cfg` instead of
-trying to do it manually with this option.
-
-You can use the instruction like this:
-
-```rust,no_run
-// build.rs
-println!("cargo::rustc-check-cfg=cfg(foo, bar)");
-```
-
-```
-cargo check -Z unstable-options -Z check-cfg
-```
-
 ## codegen-backend
 
 The `codegen-backend` feature makes it possible to select the codegen backend used by rustc using a profile.