docs: add API reference to UserPromptNode (#2052)

Author: Kludex

pydantic_ai_slim/pydantic_ai/_agent_graph.py

@@ -141,6 +141,8 @@ def is_agent_node(
 
 @dataclasses.dataclass
 class UserPromptNode(AgentNode[DepsT, NodeRunEndT]):
+    """The node that handles the user prompt and instructions."""
+
     user_prompt: str | Sequence[_messages.UserContent] | None
 
     instructions: str | None
@@ -293,7 +295,7 @@ async def add_mcp_server_tools(server: MCPServer) -> None:
 
 @dataclasses.dataclass
 class ModelRequestNode(AgentNode[DepsT, NodeRunEndT]):
-    """Make a request to the model using the last message in state.message_history."""
+    """The node that makes a request to the model using the last message in state.message_history."""
 
     request: _messages.ModelRequest
 
@@ -412,7 +414,7 @@ def _finish_handling(
 
 @dataclasses.dataclass
 class CallToolsNode(AgentNode[DepsT, NodeRunEndT]):
-    """Process a model response, and decide whether to end the run or make a new request."""
+    """The node that processes a model response, and decides whether to end the run or make a new request."""
 
     model_response: _messages.ModelResponse