[Doc] Update max_tokens to max_completion_tokens in all docs (#6248)

### What this PR does / why we need it?

Fix:

```
DeprecationWarning: max_tokens is deprecated in favor of the max_completion_tokens field.
```

- vLLM version: v0.14.1
- vLLM main:
d68209402d

Signed-off-by: shen-shanshan <467638484@qq.com>

docs/source/tutorials/pd_disaggregation_mooncake_single_node.md

@@ -271,7 +271,7 @@ curl http://192.0.0.1:8080/v1/chat/completions \
                 {"type": "text", "text": "What is the text in the illustrate?"}
             ]}
             ],
-        "max_tokens": 100,
+        "max_completion_tokens": 100,
         "temperature": 0
     }'
 ```