wrap Arel.sql to avoid error in doc of insert_all, upsert_all

[a5-stable]

Motivation / Background

An error will be raised when following the instructions in the document as is, so surrounded it with Arel.sql.

result = Book.insert_all! [{ name: "Rework", author_id: 1 }], returning: "id, name as new_name"
#=> Dangerous query method (method whose arguments are used as raw SQL) called: id, name as new_name. Known-safe values can be passed by wrapping them in Arel.sql(). (ArgumentError)   

Detail

Additional information

Checklist

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• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.