docs(client): make it clear what Read for Response reads

In case someone reads the documentation for `client::Response`
without having read the documentation for the `client` module
first, they might wonder what the `Read` impl actually reads.
Let's clarify it by annotating the `Read` impl.

src/client/response.rs

@@ -64,6 +64,7 @@ impl Response {
     }
 }
 
+/// Read the response body.
 impl Read for Response {
     #[inline]
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {