docs: ReAct reference (#22830)

The `ReAct` is used all across LangChain but it is not referenced
properly.
Added references to the original paper.

libs/langchain/langchain/agents/react/agent.py

@@ -24,6 +24,9 @@ def create_react_agent(
 ) -> Runnable:
     """Create an agent that uses ReAct prompting.
 
+    Based on paper "ReAct: Synergizing Reasoning and Acting in Language Models"
+    (https://arxiv.org/abs/2210.03629)
+
     Args:
         llm: LLM to use as the agent.
         tools: Tools this agent has access to.

libs/langchain/langchain/evaluation/agents/trajectory_eval_chain.py

@@ -103,6 +103,8 @@ class TrajectoryEvalChain(AgentTrajectoryEvaluator, LLMEvalChain):
 
     This chain is used to evaluate ReAct style agents by reasoning about
     the sequence of actions taken and their outcomes.
+    Based on the paper "ReAct: Synergizing Reasoning and Acting in Language Models"
+    (https://arxiv.org/abs/2210.03629)
 
     Example: