Fix BaseXCom docstrings that say passing None removes a filter

[FrankYang0529]

Why

• The BaseXCom.get_one, aget_one, get_value and aget_value docstrings say "Pass None (default) to remove the filter" for key, dag_id, task_id and map_index. The text comes from the Airflow 2 BaseXCom, where these parameters were optional. AIP-72: Handle Custom XCom Backend on Task SDK #47339 moved the methods into the Task SDK and made key, dag_id and task_id required str, but kept the text. Add async XCom accessors for async tasks #68299 copied it into the async methods.

How

• Describe key, dag_id and task_id without the None note.
• Describe map_index=None as pulling the XCom of a non-mapped task (map index -1).

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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[henry3260]

Good catch!