Explain when not to use a transactional migration

What does this MR do and why?

This MR updates the parts of the migration style guide that touch on transactions.

Specifically, it

1. moves up the section #heavy-operations-in-a-single-transaction under #atomicity for a better exposition.

2. adds a new section that summarizes when disable_ddl_transaction! should be used with additional explanation

3. adds a new section with an example SQL snippet that uses SET LOCAL lock_timeout to to illustrate the problems the lock retry helper is trying to prevent.

4. and finally adds a section on disable_statement_timeout.

MR acceptance checklist

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