Fix docs omitting asset from the Dag discovery safe-mode heuristic

[FrankYang0529]

Why

• The default safe-mode heuristic has matched airflow AND (dag OR asset) since Add "@asset" to decorate a function as a DAG and an asset #41325 (Airflow 3.0.0), but document still describes an airflow AND dag check.

How

• Reword document to: airflow and at least one of dag or asset, matched case-insensitively.

Verification

• uv run --project airflow-core pytest airflow-core/tests/unit/utils/test_file.py

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

• Yes - Claude Code

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