add top-level docs

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tokio/src/task/join_set.rs

@@ -1,3 +1,9 @@
+//! A collection of tasks spawned on a Tokio runtime.
+//!
+//! This module provides the [`JoinSet`] type, a collection which stores a set
+//! of spawned tasks and allows asynchronously awaiting the output of those
+//! tasks as they complete. See the documentation for the [`JoinSet`] type for
+//! details.
 use std::fmt;
 use std::future::Future;
 use std::pin::Pin;