doc: add missing return types in cluster.md

`worker.isConnected()` and `worker.isDead()` described their boolean
result only in prose, so the documentation tooling had no return type
for them.

Signed-off-by: Daniel Pinto <1619617+Cayan@users.noreply.github.com>
Assisted-by: a frontier reasoning model
PR-URL: #66453
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Cayan

doc/api/cluster.md

@@ -489,6 +489,8 @@ While a worker is alive, this is the key that indexes it in
 added: v0.11.14
 -->
 
+* Returns: {boolean}
+
 This function returns `true` if the worker is connected to its primary via its
 IPC channel, `false` otherwise. A worker is connected to its primary after it
 has been created. It is disconnected after the `'disconnect'` event is emitted.
@@ -499,6 +501,8 @@ has been created. It is disconnected after the `'disconnect'` event is emitted.
 added: v0.11.14
 -->
 
+* Returns: {boolean}
+
 This function returns `true` if the worker's process has terminated (either
 because of exiting or being signaled). Otherwise, it returns `false`.