Add note about internal representation of .NET runtime types (#44672)

Author: jkotas

docs/standard/native-interop/best-practices.md

@@ -400,6 +400,8 @@ Pointers to structs in definitions must either be passed by `ref` or use `unsafe
 
 ✔️ DO use the C# `sizeof()` instead of `Marshal.SizeOf<MyStruct>()` for blittable structures to improve performance.
 
+❌ DON'T depend on internal representation of struct types exposed by .NET runtime libraries unless it is explicitly documented.
+
 ❌ AVOID using classes to express complex native types through inheritance.
 
 ❌ AVOID using `System.Delegate` or `System.MulticastDelegate` fields to represent function pointer fields in structures.