[doc] backward compatibility for 0.6.4 (#11359)

Signed-off-by: youkaichao <youkaichao@gmail.com>

docs/source/getting_started/debugging.rst

@@ -86,6 +86,11 @@ If GPU/CPU communication cannot be established, you can use the following Python
     from vllm.distributed.device_communicators.pynccl import PyNcclCommunicator
 
     pynccl = PyNcclCommunicator(group=gloo_group, device=local_rank)
+    # pynccl is enabled by default for 0.6.5+,
+    # but for 0.6.4 and below, we need to enable it manually.
+    # keep the code for backward compatibility when because people
+    # prefer to read the latest documentation.
+    pynccl.disabled = False
 
     s = torch.cuda.Stream()
     with torch.cuda.stream(s):