Fix MaybeSend doc

Author: khvzak

src/types.rs

@@ -120,6 +120,7 @@ pub trait MaybeSend: Send {}
 #[cfg(feature = "send")]
 impl<T: Send> MaybeSend for T {}
 
+/// A trait that adds `Send` requirement if `send` feature is enabled.
 #[cfg(not(feature = "send"))]
 pub trait MaybeSend {}
 #[cfg(not(feature = "send"))]