Add handling of nullability in value types to docs

Author: Bart Koelman

docs/usage/resources/nullability.md

@@ -1,14 +1,37 @@
-# Nullability
+# Nullability in resources
 
-When the [nullable reference types](https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references) (NRT) compiler feature is enabled, it affects both ASP.NET ModelState validation and Entity Framework Core.
+Properties on a resource class can be declared as nullable or non-nullable. This affects both ASP.NET ModelState validation and the way Entity Framework Core generates database columns.
 
 Note that ModelState validation is turned off by default. It can be enabled in [options](~/usage/options.md#enable-modelstate-validation).
 
+# Value types
+
+When ModelState validation is enabled, non-nullable value types will **not** trigger a validation error when omitted in the request body.
+To make JsonApiDotNetCore return an error when such a property is missing on resource creation, declare it as nullable and annotate it with `[Required]`.
+
+Example:
+
+```c#
+public sealed class User : Identifiable<int>
+{
+    [Attr]
+    [Required]
+    public bool? IsAdministrator { get; set; }
+}
+```
+
+This makes EF Core generate non-nullable columns. And model errors are returned when nullable fields are omitted.
+
+# Reference types
+
+When the [nullable reference types](https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references) (NRT) compiler feature is enabled, it affects both ASP.NET ModelState validation and Entity Framework Core.
+
 ## NRT turned off
 
 When NRT is turned off, use `[Required]` on required attributes and relationships. This makes EF Core generate non-nullable columns. And model errors are returned when required fields are omitted.
 
 Example:
+
 ```c#
 public sealed class Label : Identifiable<int>
 {