Document Behavior Changes

Update documentation every time code behavior changes or new public APIs are introduced.

Apply this checklist: 1) Docstring accuracy: If you change semantics (error handling, return values, shutdown behavior, etc.), update the docstring to match reality. 2) Public API versioning: For any new/changed public method, add the appropriate Sphinx directive (.. versionadded:: X.Y.Z / .. versionchanged:: X.Y.Z) consistently (including any related backend/base/async docstrings). 3) User-facing docs: If behavior impacts users (e.g., shutdown semantics, retries, connection handling), update the relevant documentation section and include a version note. 4) Interface/contract documentation: For new classes/steps that implement an interface, add inline documentation describing the contract/expected behavior. 5) Docs tooling for optional deps: Prefer Sphinx mechanisms like autodoc_mock_imports for optional third-party dependencies so API docs build without hiding misconfiguration.

Example (public API versioning + docstring alignment):

class BaseBackend:
    def exists(self, task_id: str) -> bool:
        """Return True if a result exists for the given task id."""
        # ...

        .. versionadded:: 5.7.0

Example (doc update for shutdown semantics):

• If code changes shutdown to cancel pending work, ensure the worker docs’ “Warm Shutdown” section describes the new behavior and includes .. versionchanged:: X.Y.