Reference doc on cfg does not cover cfg(feature = "...")

[steveklabnik]

Migrated from rust-lang/rust#34803

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Rust appears to support #[cfg(feature = "foo")] to control optional features: Cargo doc.

The Rust reference on cfg does not mention feature: nightly doc.

[Havvy]

The conditional compilation section needs to add the feature cfg variant. They are used mostly by cargo and there isn't a language feature for turning on and off features, so would have to say something like "The compiler decides which features are enabled and which aren't." But that wording won't work because "the compiler" currently means rustc always.