Document snapshot model initialization breaking change

[AndriySvyryd]

See dotnet/efcore#22031

[AndriySvyryd]

var snapshotModel = migrationsAssembly.ModelSnapshot?.Model;
if (snapshotModel != null)
{
    snapshotModel = context.GetService<IModelRuntimeInitializer>().Initialize(snapshotModel);
}

context.GetService<IMigrationsModelDiffer>().HasDifferences(
    snapshotModel?.GetRelationalModel(),
    context.GetService<IDesignTimeModel>().Model.GetRelationalModel());