macros: render more comprehensible documentation for select! macro

tokio/src/macros/select.rs

@@ -396,8 +396,27 @@
 ///     }
 /// }
 /// ```
+#[cfg(doc)]
 #[macro_export]
 #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
+macro_rules! select {
+    {
+        $(
+            biased;
+        )?
+        $(
+            $bind:pat = $fut:expr $(, if $cond:expr)? => $handler:expr /*optional comma*/,
+        )*
+        $(
+            else => $els:expr $(,)?
+        )?
+    } => {
+        unimplemented!()
+    };
+}
+
+#[cfg(not(doc))]
+#[macro_export]
 macro_rules! select {
     // Uses a declarative macro to do **most** of the work. While it is possible
     // to implement fully with a declarative macro, a procedural macro is used