doc: add missing return types in cluster.md

[Cayan]

worker.isConnected() and worker.isDead() describe their boolean result only in prose, so the doc tooling has no return type for them. This adds * Returns: {boolean} to both, the same way #65307 did for fs.md.

Both return this.process.connected and an exit/signal check (lib/internal/cluster/worker.js). I checked on v24 that they return true/false while the worker runs and false/true after it exits.

Written with AI assistance (see the Assisted-by trailer); I reviewed the change and ran the check above myself.

[github-actions]

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