[Doc] Upgrade env VLLM_ASCEND_ENABLE_FUSED_MC2 used in nightly test and tutorials (#8441)

### What this PR does / why we need it?
The env `VLLM_ASCEND_ENABLE_FUSED_MC2` should only enabled in the
decoder node during Prefill-Decode Disaggregation scenario

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

tests/e2e/nightly/multi_node/config/Qwen3-235B-disagg-pd.yaml

@@ -10,7 +10,6 @@ env_common:
   OMP_PROC_BIND: false
   OMP_NUM_THREADS: 1
   VLLM_ASCEND_ENABLE_FLASHCOMM1: 1
-  VLLM_ASCEND_ENABLE_FUSED_MC2: 2
   TASK_QUEUE_ENABLE: 1
   SERVER_PORT: 8080
   
@@ -21,6 +20,9 @@ disaggregated_prefill:
 
 deployment:
   -
+    envs:
+      # should disable this in the prefiller node
+      VLLM_ASCEND_ENABLE_FUSED_MC2: 0
     server_cmd: >
         vllm serve "Qwen/Qwen3-235B-A22B"
         --host 0.0.0.0
@@ -57,6 +59,8 @@ deployment:
         }'
 
   -
+    envs:
+      VLLM_ASCEND_ENABLE_FUSED_MC2: 2
     server_cmd: >
         vllm serve "Qwen/Qwen3-235B-A22B"
         --host 0.0.0.0