[Doc] Update v0.8.4 release note, add contents for structured output feature (#576)

### What this PR does / why we need it?
Update v0.8.4 release note:

- Add contents for structured output feature.
- Remove redundant `(` in spec decoding.

### Does this PR introduce _any_ user-facing change?
NO

### How was this patch tested?
Preview

Signed-off-by: shen-shanshan <467638484@qq.com>

docs/source/user_guide/release_notes.md

@@ -14,7 +14,8 @@ This is the first release candidate of v0.8.4 for vllm-ascend. Please follow the
 
 - The Ascend scheduler is added for V1 engine. This scheduler is more affinity with Ascend hardware. More scheduler policy will be added in the future. [#543](https://github.com/vllm-project/vllm-ascend/pull/543)
 - Disaggregated Prefill feature is supported. Currently only 1P1D works. NPND is under design by vllm team. vllm-ascend will support it once it's ready from vLLM. Follow the [official guide](https://docs.vllm.ai/en/latest/features/disagg_prefill.html) to use. [#432](https://github.com/vllm-project/vllm-ascend/pull/432)
-- Spec decode feature works now. Currently it's only work on V0 engine. V1 engine support will come soon. [#500]((https://github.com/vllm-project/vllm-ascend/pull/500)
+- Spec decode feature works now. Currently it's only work on V0 engine. V1 engine support will come soon. [#500](https://github.com/vllm-project/vllm-ascend/pull/500)
+- Structured output feature works now on V1 Engine. Currently it only supports xgrammar backend while using guidance backend may get some errors. [#555](https://github.com/vllm-project/vllm-ascend/pull/555)
 
 ### Other