Clarify default Interpreter NNAPI behavior

PiperOrigin-RevId: 325507160
Change-Id: Id2063801d286ebda56e0105c806aaed52a930e72

Author: jdduke

tensorflow/lite/interpreter.h

@@ -364,7 +364,11 @@ class Interpreter {
   /// Returns status of success or failure.
   TfLiteStatus Invoke();
 
-  /// Enable or disable the NN API (true to enable)
+  /// Enable or disable NNAPI (true to enable). Disabled by default.
+  ///
+  /// WARNING: NNAPI cannot be disabled after the graph has been prepared
+  /// (via `AllocateTensors`) with NNAPI enabled.
+  ///
   /// NOTE: This API is deprecated, prefer using the NNAPI delegate directly.
   /// This method will be removed in a future release.
   void UseNNAPI(bool enable);