Fix reversed direction examples in FanOutMapper docs

[Lee-W]

Why

Window.Direction.FORWARD (the default) yields the period starting at the upstream key — the period it represents — while BACKWARD yields the trailing period ending at the key. The docstrings described it the other way around: they claimed FORWARD produced the trailing/"next" period, and the code examples set direction=Window.Direction.FORWARD to get the trailing window. An author following either docstring would pick the opposite direction from what they intended.

What

• Rewrote the direction explanation in airflow-core/src/airflow/partition_mappers/temporal.py and task-sdk/src/airflow/sdk/definitions/partition_mappers/temporal.py to state that FORWARD starts at the upstream key and BACKWARD trails it.
• Fixed the code examples to use Window.Direction.BACKWARD for the trailing period, and corrected the accompanying comments (e.g. "the 7 days the upstream Monday represents" vs. "the 7 days ending at the upstream Monday").

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