docs: add samples for ArimiaPlus time_series_id_col feature

[tswast]

BEGIN_COMMIT_OVERRIDE
docs: add samples for ArimaPlus time_series_id_col feature (#1577)
END_COMMIT_OVERRIDE

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• samples/snippets/create_multiple_timeseries_forecasting_model_test.py:123, tag bigquery_dataframes_bqml_arima_multiple_step_4_evaluate
• samples/snippets/create_multiple_timeseries_forecasting_model_test.py:139, tag bigquery_dataframes_bqml_arima_multiple_step_5_coefficients
• samples/snippets/create_multiple_timeseries_forecasting_model_test.py:152, tag bigquery_dataframes_bqml_arima_multiple_step_6_forecast

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[tswast]

Thanks for taking a look @rey-esp ! I'll wait for the samples checks to pass before marking this as "auto-merge".