Add doc for qwen3 next (#15337)

docs/basic_usage/qwen3.md

@@ -13,6 +13,12 @@ python3 -m sglang.launch_server --model Qwen/Qwen3-Next-80B-A3B-Instruct --tp 4
 ### Configuration Tips
 - `--max-mamba-cache-size`: Adjust `--max-mamba-cache-size` to increase mamba cache space and max running requests capability. It will decrease KV cache space as a trade-off. You can adjust it according to workload.
 - `--mamba-ssm-dtype`: `bfloat16` or `float32`, use `bfloat16` to save mamba cache size and `float32` to get more accurate results. The default setting is `float32`.
+- `--mamba-full-memory-ratio`: The ratio of mamba state memory to full kv cache memory. The default is 0.9.
+
+### Mamba Radix Cache
+SGLang supports prefix caching for Qwen3-Next models named `MambaRadixCache`, which improves inference speed by reusing computation results. There are two versions of `MambaRadixCache`:
+- `no_buffer`: The default version, which is also other hybrid linear models' choice. When it is enabled, SGLang will automatically close overlap schedule for compatibility reasons.
+- `extra_buffer`: An optimized version that is compatible with features like page size > 1, overlap schedule, and speculative decoding. It also supports storing mamba state in branching positions. However, it requires two extra mamba spaces for a ping-pong buffer for each request. To enable it, add the argument `--mamba-scheduler-strategy extra_buffer` when launching the server.
 
 ### EAGLE Speculative Decoding
 **Description**: SGLang has supported Qwen3-Next models with [EAGLE speculative decoding](https://docs.sglang.io/advanced_features/speculative_decoding.html#EAGLE-Decoding).