task: fix unstable API documentation notes (#4503)

## Motivation

PR #4499 made the `JoinSet` API unstable, but did not add a
documentation note explaining unstable features. In general, since the
docs.rs build includes unstable APIs, it's probably worth including
these notes so that users understand what it means for an API to be
unstable.

## Solution

This branch adds a note on unstable APIs to the `JoinSet` type-level
documentation, similar to the notes for `task::Builder` and the runtime
metrics APIs.

Also, I noticed that there was a broken link to the top-level
documentation on unstable APIs in the docs for `task::Builder`, so I
fixed that as well.

tokio/src/task/builder.rs

@@ -49,7 +49,7 @@ use std::future::Future;
 ///     }
 /// }
 /// ```
-/// [unstable API]: crate#unstable-features
+/// [unstable]: crate#unstable-features
 /// [`name`]: Builder::name
 /// [`spawn_local`]: Builder::spawn_local
 /// [`spawn`]: Builder::spawn

tokio/src/task/join_set.rs

@@ -17,6 +17,10 @@ use crate::util::IdleNotifiedSet;
 ///
 /// When the `JoinSet` is dropped, all tasks in the `JoinSet` are immediately aborted.
 ///
+/// **Note**: This is an [unstable API][unstable]. The public API of this type
+/// may break in 1.x releases. See [the documentation on unstable
+/// features][unstable] for details.
+///
 /// # Examples
 ///
 /// Spawn multiple tasks and wait for them.
@@ -42,6 +46,8 @@ use crate::util::IdleNotifiedSet;
 ///     }
 /// }
 /// ```
+///
+/// [unstable]: crate#unstable-features
 pub struct JoinSet<T> {
     inner: IdleNotifiedSet<JoinHandle<T>>,
 }