docs: tighten metric_stability docs to match the implementation

[joostboon]

Checked docs/data-tests/metric-stability.mdx against test_metric_stability.sql and metric_stability_query.sql on dbt-data-reliability master (5d22e8d9). The page is accurate; this fixes three spots where it was looser than the code.

• backfill_days: on incremental models and sources, a value below the window derived from min_bucket_age raises a compile error (resolve_metric_stability_window); the page only said it reduces coverage. Also documents that force_metrics_backfill re-measures the full days_back window.
• timestamp_column: can be inherited from the model's Elementary config (get_test_argument), not only set on the test.
• Metric history: stored with collected_by="metric_stability" in metric_properties, so it is not shared with anomaly tests' metrics. Replaced "reusing Elementary's metric caching", which suggested otherwise.

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