docs(bigquery-connection): supply a manual connection_id to assist with cleanup

[tswast]

Description

Fixes #11720

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Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [N/A] README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• [N/A] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [N/A] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [N/A] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [N/A] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[tswast]

Note: When you look at the cloud console now, the connections have been cleaned up because I added some generic cleanup code in

https://github.com/googleapis/python-bigquery-dataframes/blob/57ccabcd1402b7938e2c7068e5b4880ef018f39c/samples/snippets/remote_function_test.py#L24-L50

but I'd like to remove that code once the source of the stray connections has been fixed.