Feature gate boolean lit support in cfg predicates

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diff --git a/src/doc/unstable-book/src/language-features/cfg-boolean-literals.md b/src/doc/unstable-book/src/language-features/cfg-boolean-literals.md
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+# `cfg_boolean_literals`
+
+The tracking issue for this feature is: [#131204]
+
+[#131204]: https://github.com/rust-lang/rust/issues/131204
+
+------------------------
+
+The `cfg_boolean_literals` feature makes it possible to use the `true`/`false`
+literal as cfg predicate. They always evaluate to true/false respectively.
+
+## Examples
+
+```rust
+#![feature(cfg_boolean_literals)]
+
+#[cfg(true)]
+const A: i32 = 5;
+
+#[cfg(all(false))]
+const A: i32 = 58 * 89;
+```