added EncryptedAsyncWrite::{flush,shutdown}() TODOs

src/stream.rs

@@ -137,10 +137,10 @@ impl<'a, S: AsyncWrite> AsyncWrite for EncryptedWriteHalf<'a, S>
 	poll
     }
     fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-	todo!()
+	todo!("TODO: Copy the impl of chacha20stream's `AsyncSink::flush()`")
     }
     fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-	todo!()
+	todo!("TODO: Copy the impl of chacha20stream's `AsyncSink::shutdown()`")
     }
 }
 //TODO: EncryptedWriteHalf's AsyncWrite impl should en/decrypt the input buffer into `crypt_buffer` then send it to `backing`.