Document what feature everything requires
diff --git a/src/file.rs b/src/file.rs
index 5875348..357cf6e 100644
--- a/src/file.rs
+++ b/src/file.rs
@@ -10,6 +10,8 @@
 
 ast_struct! {
     /// A complete file of Rust source code.
+    ///
+    /// *This type is available if Syn is built with the `"full"` feature.*
     pub struct File {
         pub shebang: Option<String>,
         pub attrs: Vec<Attribute>,