I think we should avoid statements like "seems to work" in the official docs, either it works or it doesn't ;) Its an issue that different parts of SilverStripe (MySQL vs Postgres vs. SQL Server, Versioned) accept different index formats, but we should solve that by consistent docs (you've done that above, great!). Could you remove this paragraph please?

Do you maybe have time to implement warnings in DataObject when using `$indexes` with string values, rather than boolean `true` or an array?