Document 6.0.0 may break `FactoryBot.lint`

[choznerol]

In a sufficiently large codebase, the automatic definition of traits for Active Record enum (#1380) may result in more combinations being checked by FactoryBot.lint. This breaking change may not be obvious because the first impression of the error messages may be "these factories were passing before the upgrade but now failing ?", while the truth is that they don't exist before the upgrade. It took me a while to realize this so I hope adding this note could help people be aware of this situation when upgrading.