docs(child-pool): add JSDoc to ChildPool public methods

[yogeshwaran-c]

Summary

• Adds concise JSDoc to public methods of ChildPool so downstream users get IDE hints and parameter help.
• Documentation only — no runtime or API changes.

Test plan

• No code logic changed; only comments added
• npm run build passes locally

[manast]

This is a private class, and thus we need to mark it as so, otherwise users will get confused believing they can also use this functionality which is strictly internal for BullMQ.