rendering: document managed srp v1 boundaries

managed/XCEngine.ScriptCore/Rendering/Core/DirectionalShadowExecutionContext.cs

@@ -1,5 +1,8 @@
 namespace XCEngine.Rendering
 {
+    // SRP v1 shadow hook only configures whether the native main directional
+    // shadow execution should run. It does not expose a custom managed shadow
+    // graph or scene backend.
     public sealed class DirectionalShadowExecutionContext
     {
         private readonly ulong m_nativeHandle;