libbinder_ndk: docs for AIBinder_debugGetRefCount

\return doc was missing

Bug: 111445392
Test: N/A
Change-Id: Ie7a2b7a12244386cabcda04ef782fd953c7d6086
diff --git a/libs/binder/ndk/include_ndk/android/binder_ibinder.h b/libs/binder/ndk/include_ndk/android/binder_ibinder.h
index 5222bda..d711ad8 100644
--- a/libs/binder/ndk/include_ndk/android/binder_ibinder.h
+++ b/libs/binder/ndk/include_ndk/android/binder_ibinder.h
@@ -287,6 +287,9 @@
  * For debugging only!
  *
  * \param binder the binder object to retrieve the refcount of.
+ *
+ * \return the number of strong-refs on this binder in this process. If binder is null, this will be
+ * -1.
  */
 int32_t AIBinder_debugGetRefCount(AIBinder* binder) __INTRODUCED_IN(29);