[Doc] Update V1 user guide for multi-modality (#15460)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/source/getting_started/v1_user_guide.md

@@ -129,6 +129,9 @@ in progress.
 - **Spec Decode**: Currently, only ngram-based spec decode is supported in V1. There
   will be follow-up work to support other types of spec decode (e.g., see [PR #13933](https://github.com/vllm-project/vllm/pull/13933)). We will prioritize the support for Eagle, MTP compared to draft model based spec decode.
 
+- **Multimodal Models**: V1 is almost fully compatible with V0 except that interleaved modality input is not supported yet.
+  See [here](https://github.com/orgs/vllm-project/projects/8) for the status of upcoming features and optimizations.
+
 #### Features to Be Supported
 
 - **FP8 KV Cache**: While vLLM V1 introduces new FP8 kernels for model weight quantization, support for an FP8 key–value cache is not yet available. Users must continue using FP16 (or other supported precisions) for the KV cache.