docs: clarify cluster listener initialization

[oiahoon]

Summary

• explain that ClusterRegistry must be instantiated before branching between primary and worker processes
• document that its constructor installs the IPC listeners workers need to answer aggregation requests
• point readers to the existing cluster example that follows this pattern

Testing

• npm test (28 suites, 540 tests, 34 snapshots)
• source/example/documentation invariant assertions
• git diff --check

Fixes #747