Document `proc-macro` feature
diff --git a/src/lib.rs b/src/lib.rs
index d0e291d..10f6282 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -254,6 +254,8 @@
//! types.
//! - **`extra-traits`** — Debug, Eq, PartialEq, Hash impls for all syntax tree
//! types.
+//! - **`proc-macro`** *(enabled by default)* — Runtime dependency on the
+//! dynamic library libproc_macro from rustc toolchain.
// Syn types in rustdoc of other crates get linked to here.
#![doc(html_root_url = "https://docs.rs/syn/0.13.1")]
@@ -540,7 +542,8 @@
///
/// [`syn::parse2`]: fn.parse2.html
///
-/// *This function is available if Syn is built with the `"parsing"` feature.*
+/// *This function is available if Syn is built with both the `"parsing"` and
+/// `"proc-macro"` features.*
///
/// # Examples
///