callback.md: WaitForCallbackContext is not defined

The `waitForCallback` section in `docs/sdk-reference/operations/callback.md` references `WaitForCallbackContext` as a parameter type but never defines it.

There is no heading, field list, or description explaining what the context provides. At minimum it should document that it has a `logger` (same as `StepContext`), and any other fields or methods it exposes.

Comparable: `StepContext` has its own `### StepContext` subsection in `step.md`.