Change UnixListener::poll_accept to public

This makes it consistent with `TcpListener::poll_accept` and other
public poll methods the library provides. This particular method is
useful for writing generic code that accepts connections since async
functions can't easily be used with traits. It is possible to
generically accept connections with `Incoming`, however, this doesn't
return the incoming `SocketAddr`.

tokio/src/net/unix/listener.rs

@@ -104,7 +104,8 @@ impl UnixListener {
         poll_fn(|cx| self.poll_accept(cx)).await
     }
 
-    pub(crate) fn poll_accept(
+    #[doc(hidden)]
+    pub fn poll_accept(
         &mut self,
         cx: &mut Context<'_>,
     ) -> Poll<io::Result<(UnixStream, SocketAddr)>> {