docs: add explain forecast snippet to multiple time series tutorial

[rey-esp]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/create_multiple_timeseries_forecasting_model_test.py:164, tag bigquery_dataframes_bqml_arima_multiple_step_7_explain

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[tswast]

Doctest failure:

FAILED third_party/bigframes_vendored/geopandas/geoseries.py::geoseries.GeoSeries.boundary

Appears to be unrelated to this change.