Document that required columns are created nullable when table splitting

See https://github.com/dotnet/efcore/issues/27516

The issue here is that our guidance is to introduce a required property in optional dependents. However, when table splitting, which is the default for owned types, Migrations will create a nullable column with a null default for this property. This makes all existing dependents effectively disappear. We need to document that the existing rows need to be populated with a non-null value so that they are detected as existing.