Document the sink flow lifecycle contract

Key sink lifecycle methods lacked API-level guidance about ordering and responsibility, making restore, checkpoint, writer creation, and resource cleanup behavior harder for contributors to reason about safely.

Added focused Javadocs to the remaining lifecycle methods, describing initialization, restored writer creation, record handling, checkpoint barrier processing, and cleanup without changing runtime behavior or public APIs.

• Makes the sink execution contract easier to understand for connector and engine contributors.
• Reduces lifecycle misuse around restore, checkpoint boundaries, and cleanup responsibilities.
• Improves maintainability without introducing runtime or compatibility risk.

• Verified formatting and dependency-aware compilation through a 53-module reactor build.
• Confirmed the change is isolated to one engine lifecycle source file.
• Received approvals from two maintainers; the authored merge commit is GitHub-verified.