[Docs] Update docs to include mm processor + encoder benchmarks (#34083)

Signed-off-by: Reagan <reaganjlee@gmail.com>

docs/cli/bench/mm_processor.md

@@ -1,5 +1,51 @@
 # vllm bench mm-processor
 
+## Overview
+
+`vllm bench mm-processor` profiles the multimodal input processor pipeline of
+vision-language models. It measures per-stage latency from the HuggingFace
+processor through to the encoder forward pass, helping you identify
+preprocessing bottlenecks and understand how different image resolutions or
+item counts affect end-to-end request time.
+
+The benchmark supports two data sources: synthetic random multimodal inputs
+(`random-mm`) and HuggingFace datasets (`hf`). Warmup requests are run before
+measurement to ensure stable results.
+
+## Quick Start
+
+```bash
+vllm bench mm-processor \
+  --model Qwen/Qwen2-VL-7B-Instruct \
+  --dataset-name random-mm \
+  --num-prompts 50 \
+  --random-input-len 300 \
+  --random-output-len 40 \
+  --random-mm-base-items-per-request 2 \
+  --random-mm-limit-mm-per-prompt '{"image": 3, "video": 0}' \
+  --random-mm-bucket-config '{(256, 256, 1): 0.7, (720, 1280, 1): 0.3}'
+```
+
+## Measured Stages
+
+| Stage | Description |
+|-------|-------------|
+| `get_mm_hashes_secs` | Time spent hashing multimodal inputs |
+| `get_cache_missing_items_secs` | Time spent looking up the processor cache |
+| `apply_hf_processor_secs` | Time spent in the HuggingFace processor |
+| `merge_mm_kwargs_secs` | Time spent merging multimodal kwargs |
+| `apply_prompt_updates_secs` | Time spent updating prompt tokens |
+| `preprocessor_total_secs` | Total preprocessing time |
+| `encoder_forward_secs` | Time spent in the encoder model forward pass |
+| `num_encoder_calls` | Number of encoder invocations per request |
+
+The benchmark also reports end-to-end latency (TTFT + decode time) per
+request. Use `--metric-percentiles` to select which percentiles to report
+(default: p99) and `--output-json` to save results.
+
+For more examples (HF datasets, warmup, JSON output), see
+[Benchmarking CLI — Multimodal Processor Benchmark](../../benchmarking/cli.md#multimodal-processor-benchmark).
+
 ## JSON CLI Arguments
 
 --8<-- "docs/cli/json_tip.inc.md"