net: make UnixListener::poll_accept public (#2880)

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/tcp/listener.rs

@@ -185,10 +185,10 @@ impl TcpListener {
         poll_fn(|cx| self.poll_accept(cx)).await
     }
 
-    /// Attempts to poll `SocketAddr` and `TcpStream` bound to this address.
+    /// Polls to accept a new incoming connection to this listener.
     ///
-    /// In case if I/O resource isn't ready yet, `Poll::Pending` is returned and
-    /// current task will be notified by a waker.
+    /// If there is no connection to accept, `Poll::Pending` is returned and
+    /// the current task will be notified by a waker.
     pub fn poll_accept(
         &mut self,
         cx: &mut Context<'_>,

tokio/src/net/unix/listener.rs

@@ -104,7 +104,11 @@ impl UnixListener {
         poll_fn(|cx| self.poll_accept(cx)).await
     }
 
-    pub(crate) fn poll_accept(
+    /// Polls to accept a new incoming connection to this listener.
+    ///
+    /// If there is no connection to accept, `Poll::Pending` is returned and
+    /// the current task will be notified by a waker.
+    pub fn poll_accept(
         &mut self,
         cx: &mut Context<'_>,
     ) -> Poll<io::Result<(UnixStream, SocketAddr)>> {