Document Pool close and terminate behavior

[Mirochill]

Description

Clarifies the Pool shutdown docs so users can tell when to use close/join versus terminate:

• explains that close prevents new jobs and lets workers drain their queues before join returns
• explains that terminate stops workers immediately and may leave pending results incomplete
• adds the same guidance to the user guide, not only the generated API reference

Fixes: #88

Validation

Not run locally (documentation-only change).