Show that derives are specific to feature="derive" in documentation

Author: dtolnay

Diff for: serde/src/lib.rs

@@ -326,6 +326,7 @@ extern crate serde_derive;
 
 /// Derive macro available if serde is built with `features = ["derive"]`.
 #[cfg(feature = "serde_derive")]
+#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
 pub use serde_derive::{Deserialize, Serialize};
 
 #[cfg(all(not(no_serde_derive), any(feature = "std", feature = "alloc")))]