docs(types): add info about nullableByDefault option

docs/types-and-fields.md

@@ -54,9 +54,11 @@ For simple types (like `string` or `boolean`) this is all that's needed but due
 
 Why use function syntax and not a simple `{ type: Rate }` config object? Because, by using function syntax we solve the problem of circular dependencies (e.g. Post <--> User), so it was adopted as a convention. You can use the shorthand syntax `@Field(() => Rate)` if you want to save some keystrokes but it might be less readable for others.
 
-For nullable properties like `averageRating` which might not be defined when a recipe has no ratings yet, we mark the class property as optional with a `?:` operator and also have to pass the `{ nullable: true }` decorator parameter. We should be aware that when we declare our type as a nullable union (e.g. `string | null`), we need to explicitly provide the type to the `@Field` decorator.
+By default, all fields are non nullable, just like properties in TypeScript. However, you can change that behavior by providing `nullableByDefault: true` option in `buildSchema` settings, described in [bootstrap guide](./bootstrap.md).
 
-In the case of lists, we may also need to define their nullability in a more detailed form. The basic `{ nullable: true | false }` setting only applies to the whole list (`[Item!]` or `[Item!]!`), so if we need a sparse array, we can control the list items' nullability via `nullable: items` (for `[Item]!`) or `nullable: itemsAndList` (for the `[Item]`) option.
+So for nullable properties like `averageRating` which might not be defined when a recipe has no ratings yet, we mark the class property as optional with a `?:` operator and also have to pass the `{ nullable: true }` decorator parameter. We should be aware that when we declare our type as a nullable union (e.g. `string | null`), we need to explicitly provide the type to the `@Field` decorator.
+
+In the case of lists, we may also need to define their nullability in a more detailed form. The basic `{ nullable: true | false }` setting only applies to the whole list (`[Item!]` or `[Item!]!`), so if we need a sparse array, we can control the list items' nullability via `nullable: items` (for `[Item]!`) or `nullable: itemsAndList` (for the `[Item]`) option. Be aware that setting `nullableByDefault: true` option will also apply to lists, so it will produce `[Item]` type, just like with `nullable: itemsAndList`.
 
 In the config object we can also provide the `description` and `deprecationReason` properties for GraphQL schema purposes.