[doc] feat: add mtp spec log (verl-project#5491)

### What does this PR do?

Enabling MTP improves the rollout acceptance rate by around 14%.
However, on H20 GPUs, overall throughput does not increase and even
decreases slightly.

add docs.

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Author: ArronHZG

docs/advance/mtp.md

@@ -71,6 +71,11 @@ It is recommended to adopt the `detach_encoder=True` approach for MTP training.
 
 # 4. Performance Notes for MTP in Rollout Inference
 
+Enabling MTP improves the rollout acceptance rate by around 14%. However, on H20 GPUs, overall throughput does not increase and even decreases slightly.
+
+![spec_log](
+https://github.com/ArronHZG/verl-community/blob/main/docs/spec_log.png?raw=true)
+
 The effectiveness of MTP-accelerated Rollout is significantly affected by **model size** and **inference hardware**. Key reference information is as follows:
 
 **Hardware Tensor Core Performance**