docs: document single-argument link_error errback used by send_task (#10427)

* docs: document single-argument link_error errback used by send_task

The Linking (callbacks/errbacks) guide only documented the direct (request, exc, traceback) errback convention. Celery also applies an errback as a task with the failed task's id as the sole argument when the errback takes a single argument or is not registered on the worker handling the failure (backends/base.py) - which is what happens with app.send_task, since the task is referenced by name. Document this single-argument form and the allow_join_result + AsyncResult(task_id) pattern, and soften the blanket claim that errbacks are never called as a task.

Closes #9034

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Co-authored-by: Asif Saif Uddin {"Auvi":"অভি"} <auvipy@gmail.com>