docs: sync Haystack API reference on Docusaurus

docs-website/reference/haystack-api/generators_api.md

@@ -1221,8 +1221,8 @@ __init__(
     *,
     chat_generator: ChatGenerator,
     system_prompt: str | None = None,
-    user_prompt: str | None = None,
-    required_variables: list[str] | Literal["*"] | None = None,
+    user_prompt: str,
+    required_variables: list[str] | Literal["*"] = "*",
     streaming_callback: StreamingCallbackT | None = None
 ) -> None
 ```
@@ -1233,12 +1233,18 @@ Initialize the LLM component.
 
 - **chat_generator** (<code>ChatGenerator</code>) – An instance of the chat generator that the LLM should use.
 - **system_prompt** (<code>str | None</code>) – System prompt for the LLM.
-- **user_prompt** (<code>str | None</code>) – User prompt for the LLM. If provided this is appended to the messages provided at runtime.
-- **required_variables** (<code>list\[str\] | Literal['\*'] | None</code>) – List variables that must be provided as input to user_prompt.
+- **user_prompt** (<code>str</code>) – User prompt for the LLM. Must contain at least one Jinja2 template variable
+  (e.g., `{{ variable_name }}`). This prompt is appended to the messages provided at runtime.
+- **required_variables** (<code>list\[str\] | Literal['\*']</code>) – Variables that must be provided as input to user_prompt.
   If a variable listed as required is not provided, an exception is raised.
-  If set to `"*"`, all variables found in the prompt are required. Optional.
+  If set to `"*"`, all variables found in the prompt are required. Defaults to `"*"`.
 - **streaming_callback** (<code>StreamingCallbackT | None</code>) – A callback that will be invoked when a response is streamed from the LLM.
 
+**Raises:**
+
+- <code>ValueError</code> – If user_prompt contains no template variables.
+- <code>ValueError</code> – If required_variables is an empty list.
+
 #### to_dict
 
 ```python