[Docs] Sync docs_new with legacy docs and update migration redirects (#23337)

Co-authored-by: Mingyi <wisclmy0611@gmail.com>

docs_new/docs/developer_guide/bench_serving.mdx

@@ -142,7 +142,7 @@ python3 -m sglang.bench_serving \
 
 - `--output-file FILE.jsonl`: append JSONL results to file; auto-named if unspecified
 - `--output-details`: include per-request arrays (generated texts, errors, ttfts, itls, input/output lens)
-- `--extra-request-body '{"top_p":0.9,"temperature":0.6}'`: merged into payload (sampling params, etc.)
+- `--extra-request-body '{"top_p":0.9,"temperature":0.6}'`: merged into payload (sampling params, etc.)
 - `--disable-ignore-eos`: pass through EOS behavior (varies by backend)
 - `--warmup-requests N`: run warmup requests with short output first (default 1)
 - `--flush-cache`: call `/flush_cache` (sglang) before main run
@@ -335,7 +335,7 @@ python3 -m sglang.bench_serving \
 python3 -m sglang.bench_serving \
   --backend sglang \
   --host 127.0.0.1 --port 30000 \
-  --model mode-name \
+  --model model-name \
   --dataset-name mooncake \
   --mooncake-slowdown-factor 1.0 \
   --mooncake-num-rounds 1000 \
@@ -344,6 +344,41 @@ python3 -m sglang.bench_serving \
   --random-output-len 256
 ```
 
+10) Fake decode stress testing (PD disaggregation, decode-only):
+
+When benchmarking pure decode performance in a PD disaggregation setup, you can bypass the prefill node entirely by using `--fake-prefill`. This requires the decode server to be started with `--disaggregation-transfer-backend fake`:
+
+```bash Command
+# Step 1: Start a decode-only server with fake transfer backend
+python -m sglang.launch_server \
+  --model-path meta-llama/Llama-3.1-8B-Instruct \
+  --disaggregation-mode decode \
+  --disaggregation-transfer-backend fake \
+  --port 30001
+
+# Step 2: Run bench_serving with --fake-prefill
+python3 -m sglang.bench_serving \
+  --backend sglang \
+  --host 127.0.0.1 --port 30001 \
+  --model meta-llama/Llama-3.1-8B-Instruct \
+  --dataset-name random \
+  --num-prompts 500 \
+  --random-input-len 1024 --random-output-len 256 \
+  --fake-prefill
+```
+
+Similarly, `bench_one_batch_server` also supports `--fake-prefill`:
+
+```bash Command
+python3 -m sglang.bench_one_batch_server \
+  --base-url http://127.0.0.1:30001 \
+  --model-path meta-llama/Llama-3.1-8B-Instruct \
+  --batch-size 32 --input-len 1024 --output-len 256 \
+  --fake-prefill
+```
+
+The `--fake-prefill` flag automatically injects special sentinel values into each request, telling the decode server to skip real KV transfer and generate fake KV data locally.
+
 ### Troubleshooting
 
 - All requests failed: verify `--backend`, server URL/port, `--model`, and authentication. Check warmup errors printed by the script.
@@ -355,4 +390,4 @@ python3 -m sglang.bench_serving \
 ### Notes
 
 - The script raises the file descriptor soft limit (`RLIMIT_NOFILE`) to help with many concurrent connections.
-- For sglang, `/get_server_info` is queried post-run to report speculative decoding accept length when available.
+- For sglang, `/server_info` is queried post-run to report speculative decoding accept length when available.