Add "Nested Transactions" section

[wata727]

See also rubocop/rubocop-rails#1236

[pirj]

I didn’t look close yet, but seems interested at a glance https://about.gitlab.com/blog/2021/09/29/why-we-spent-the-last-month-eliminating-postgresql-subtransactions/

[wata727]

Thank you for sharing this article. This article seems to be referring to performance issues caused by nested transactions in PostgreSQL.

Please note that this PR is not about whether you should use nested transactions, but only that if you do, you should not use joinable: false. This is a fairly partial mention, so I'm honestly not sure if it should be added to the style guide.