Small update for external_launcher backend docs (#14288)

Author: zhe-thoughts

vllm/executor/uniproc_executor.py

@@ -103,6 +103,7 @@ def _init_executor(self) -> None:
         # so we can use the env:// method.
         # required env vars:
         # - RANK
+        # - LOCAL_RANK
         # - MASTER_ADDR
         # - MASTER_PORT
         distributed_init_method = "env://"