service: Call inner.poll_ready() in docs when cloning inner (tower-rs…

…#679)

- The documentation should call self.inner.poll_ready() in the
  Wrapper::poll_ready() call to emphasize that self.inner may only be
  ready on the original instance and not a clone of inner.

tower-service/src/lib.rs

@@ -260,7 +260,7 @@ use std::task::{Context, Poll};
 /// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
 ///
 /// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-/// Poll::Ready(Ok(()))
+/// self.inner.poll_ready(cx)
 /// }
 ///
 /// fn call(&mut self, req: R) -> Self::Future {
@@ -295,7 +295,7 @@ use std::task::{Context, Poll};
 /// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
 ///
 /// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-/// Poll::Ready(Ok(()))
+/// self.inner.poll_ready(cx)
 /// }
 ///
 /// fn call(&mut self, req: R) -> Self::Future {