ndk: ANativeActivity: Fix 'clazz' member documentation.

As discussed on android-ndk, the member is misnamed

Change-Id: I2a5a1c8dce327ddf9ab7a4d14f4e2fdf62808753

ndk/platforms/android-9/include/android/native_activity.h

@@ -60,7 +60,14 @@ typedef struct ANativeActivity {
     JNIEnv* env;
 
     /**
-     * The NativeActivity Java class.
+     * The NativeActivity object handle.
+     *
+     * IMPORTANT NOTE: This member is mis-named. It should really be named
+     * 'activity' instead of 'clazz', since it's a reference to the
+     * NativeActivity instance created by the system for you.
+     *
+     * We unfortunately cannot change this without breaking NDK
+     * source-compatibility.
      */
     jobject clazz;