docs: clarify handoff() docs, callable not supported

[ihower]

Related issue: #2115

The docs said that handoff() can accept a function that returns an agent, but this is not actually supported in the current code.

• The agent type hint does not include Callable.
• The runtime never calls a function to get an agent.
• There is no test that uses a function; all tests use Agent instances only.

This PR updates the docs to match the real behavior.

If callable support is needed, it will require more work and can be added in a separate PR in the future.

[seratch]

Thanks for sending this document update!