[docs] Add note that ref struct can't be converted to object (#43602)

Author: pypkaed

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

@@ -28,7 +28,7 @@ C# has many built-in reference types. They have keywords or operators that are s
 
 ## The object type
 
-The `object` type is an alias for <xref:System.Object?displayProperty=nameWithType> in .NET. In the unified type system of C#, all types, predefined and user-defined, reference types and value types, inherit directly or indirectly from <xref:System.Object?displayProperty=nameWithType>. You can assign values of any type to variables of type `object`. Any `object` variable can be assigned to its default value using the literal `null`. When a variable of a value type is converted to object, it's said to be *boxed*. When a variable of type `object` is converted to a value type, it's said to be *unboxed*. For more information, see [Boxing and Unboxing](../../programming-guide/types/boxing-and-unboxing.md).
+The `object` type is an alias for <xref:System.Object?displayProperty=nameWithType> in .NET. In the unified type system of C#, all types, predefined and user-defined, reference types and value types, inherit directly or indirectly from <xref:System.Object?displayProperty=nameWithType>. You can assign values of any type (except `ref struct`, see [ref struct](ref-struct.md)) to variables of type `object`. Any `object` variable can be assigned to its default value using the literal `null`. When a variable of a value type is converted to object, it's said to be *boxed*. When a variable of type `object` is converted to a value type, it's said to be *unboxed*. For more information, see [Boxing and Unboxing](../../programming-guide/types/boxing-and-unboxing.md).
 
 ## The string type