Document AutoTransactionBehavior

[roji]

EF Core 7.0 introduced AutoTransactionBehavior to control when transactions are started by EF Core when saving changes (Never, Always, WhenNeeded).

See e.g. dotnet/efcore#31119

[voroninp]

It would be nice if you also mention this in xmldocs of IDbTransactionInterceptor. It was a WTF moment when its methods were not called, because SaveChanges decided not to create a transaction.