docs(server): set doc_cfg attributes for tcp feature gated Server methods (#2401)

src/server/server.rs

@@ -52,8 +52,10 @@ impl<I> Server<I, ()> {
     }
 }
 
-#[cfg(feature = "tcp")]
-impl Server<AddrIncoming, ()> {
+cfg_feature! {
+    #![all(feature = "tcp")]
+
+    impl Server<AddrIncoming, ()> {
         /// Binds to the provided address, and returns a [`Builder`](Builder).
         ///
         /// # Panics
@@ -76,14 +78,18 @@ impl Server<AddrIncoming, ()> {
         pub fn from_tcp(listener: StdTcpListener) -> Result<Builder<AddrIncoming>, crate::Error> {
             AddrIncoming::from_std(listener).map(Server::builder)
         }
+    }
 }
 
-#[cfg(feature = "tcp")]
-impl<S, E> Server<AddrIncoming, S, E> {
+cfg_feature! {
+    #![all(feature = "tcp")]
+
+    impl<S, E> Server<AddrIncoming, S, E> {
         /// Returns the local address that this server is bound to.
         pub fn local_addr(&self) -> SocketAddr {
             self.spawn_all.local_addr()
         }
+    }
 }
 
 impl<I, IO, IE, S, E, B> Server<I, S, E>