Doc: Update Grammar Backend (#2545)

Co-authored-by: Chayenne <zhaochen20@outlook.com>

docs/backend/openai_api_completions.ipynb

@@ -221,7 +221,14 @@
    "metadata": {},
    "source": [
     "## Structured decoding (JSON, Regex)\n",
-    "You can specify a JSON schema or a regular expression to constrain the model output. The model output will be guaranteed to follow the given constraints.\n",
+    "You can define a JSON schema or regular expression to constrain the model's output. The model output will be guaranteed to follow the given constraints and this depends on the grammar backend.\n",
+    "\n",
+    "SGlang has two backends: outlines (default) and Xgrammar. Xgrammar enhances JSON decoding performance but does not support regular expressions. To use Xgrammar, add the `--grammar-backend xgrammar` when launching the server:\n",
+    "\n",
+    "```bash\n",
+    "python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3.1-8B-Instruct \\\n",
+    "--port 30000 --host 0.0.0.0 --grammar-backend xgrammar\n",
+    "```\n",
     "\n",
     "### JSON"
    ]