Improved mutex comment

Author: RossBrunton

offload/include/Shared/APITypes.h

@@ -76,7 +76,9 @@ struct __tgt_async_info {
   /// should be freed after finalization.
   llvm::SmallVector<void *, 2> AssociatedAllocations;
 
-  /// Mutex to guard access to AssociatedAllocations and the Queue
+  /// Mutex to guard access to AssociatedAllocations and the Queue.
+  /// This is only used for liboffload and should be ignored in libomptarget
+  /// code.
   std::mutex Mutex;
 
   /// The kernel launch environment used to issue a kernel. Stored here to