Improve net API description

src/include/nccl_net.h

@@ -49,7 +49,8 @@ typedef struct {
   // Perform a flush/fence to make sure all data received with NCCL_PTR_CUDA is
   // visible to the GPU
   ncclResult_t (*flush)(void* recvComm, void* data, int size);
-  // Test whether a request is complete and return the size received (can be less than requested).
+  // Test whether a request is complete. If size is not NULL, it returns the
+  // number of bytes sent/received.
   ncclResult_t (*test)(void* request, int* done, int* size);
   // Close and free send/recv comm objects
   ncclResult_t (*closeSend)(void* sendComm);