Add note about NRT runtime behavior changes in libraries

docs/csharp/language-reference/builtin-types/nullable-reference-types.md

@@ -46,6 +46,9 @@ try
 
 The examples in the previous section illustrate the nature of nullable reference types. Nullable reference types aren't new class types, but rather annotations on existing reference types. The compiler uses those annotations to help you find potential null reference errors in your code. There's no runtime difference between a non-nullable reference type and a nullable reference type. The compiler doesn't add any runtime checking for non-nullable reference types. The benefits are in the compile-time analysis. The compiler generates warnings that help you find and fix potential null errors in your code. You declare your intent, and the compiler warns you when your code violates that intent.
 
+> [!IMPORTANT]
+> Nullable reference annotations don't introduce behavior changes, but other libraries may use reflection to produce different runtime behavior for nullable and non-nullable reference types. Notably, Entity Framework Core reads nullable attributes. It interprets a nullable reference as an optional value, and a non-nullable reference as a required value.
+
 In a nullable enabled context, the compiler performs static analysis on variables of any reference type, both nullable and non-nullable. The compiler tracks the *null-state* of each reference variable as either *not-null* or *maybe-null*. The default state of a non-nullable reference is *not-null*. The default state of a nullable reference is *maybe-null*.
 
 Non-nullable reference types should always be safe to dereference because their *null-state* is *not-null*. To enforce that rule, the compiler issues warnings if a non-nullable reference type isn't initialized to a non-null value. Local variables must be assigned where they're declared. Every field must be assigned a *not-null* value, in a field initializer or every constructor. The compiler issues warnings when a non-nullable reference is assigned to a reference whose state is *maybe-null*. Generally, a non-nullable reference is *not-null* and no warnings are issued when those variables are dereferenced.