[Doc] Added unmentioned required option "method" in the usage of EAGLE-3 based models (#21737)

Signed-off-by: Dilute-l <dilu2333@163.com>
Co-authored-by: Dilute-l <dilu2333@163.com>

docs/features/spec_decode.md

@@ -203,6 +203,7 @@ an [EAGLE (Extrapolation Algorithm for Greater Language-model Efficiency)](https
             "model": "yuhuili/EAGLE-LLaMA3-Instruct-8B",
             "draft_tensor_parallel_size": 1,
             "num_speculative_tokens": 2,
+            "method": "eagle",
         },
     )
 
@@ -231,6 +232,9 @@ A few important things to consider when using the EAGLE based draft models:
    reported in the reference implementation [here](https://github.com/SafeAILab/EAGLE). This issue is under
    investigation and tracked here: <gh-issue:9565>.
 
+4. When using EAGLE-3 based draft model, option "method" must be set to "eagle3".
+   That is, to specify `"method": "eagle3"` in `speculative_config`.
+
 A variety of EAGLE draft models are available on the Hugging Face hub:
 
 | Base Model                                                           | EAGLE on Hugging Face                     | # EAGLE Parameters |