Document what feature everything requires
diff --git a/src/lifetime.rs b/src/lifetime.rs
index 4666da6..e192c5c 100644
--- a/src/lifetime.rs
+++ b/src/lifetime.rs
@@ -24,6 +24,9 @@
/// the XID_Start property.
/// - All following characters must be Unicode code points with the XID_Continue
/// property.
+///
+/// *This type is available if Syn is built with the `"derive"` or `"full"`
+/// feature.*
#[cfg_attr(feature = "extra-traits", derive(Debug))]
#[cfg_attr(feature = "clone-impls", derive(Clone))]
pub struct Lifetime {