test taskdump docs (#8064)

tokio/src/macros/cfg.rs

@@ -520,6 +520,20 @@ macro_rules! cfg_taskdump {
                     target_arch = "x86_64"
                 )
             ))]
+            #[cfg_attr(
+                docsrs,
+                doc(cfg(all(
+                    tokio_unstable,
+                    feature = "taskdump",
+                    feature = "rt",
+                    target_os = "linux",
+                    any(
+                        target_arch = "aarch64",
+                        target_arch = "x86",
+                        target_arch = "x86_64"
+                    )
+                )))
+            )]
             $item
         )*
     };