task: fix task module feature flags

tokio/src/lib.rs

@@ -549,6 +549,11 @@ cfg_not_sync! {
     mod sync;
 }
 
+// Currently, task module does not expose any public API outside `rt`
+// feature, so we mark it in the docs. This happens only to docs to
+// avoid introducing breaking changes by restricting the visibility
+// of the task module.
+#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
 pub mod task;
 cfg_rt! {
     pub use task::spawn;