Add Buffer::new note on how to set bound (#425)

tower-buffer/src/service.rs

@@ -34,6 +34,12 @@ where
     ///
     /// The default Tokio executor is used to run the given service, which means that this method
     /// must be called while on the Tokio runtime.
+    ///
+    /// # Note on setting `bound`
+    /// When `Buffer`'s implementation of `poll_ready` returns `Poll::Ready`, it reserves a
+    /// slot in the channel for the forthcoming `call()`. However, if this call doesn't arrive,
+    /// this reserved slot may be held up for a long time. As a result, it's advisable to set
+    /// `bound` to be at least the maximum number of concurrent requests the `Buffer` will see.
     pub fn new(service: T, bound: usize) -> Self
     where
         T: Send + 'static,