add docstring for in memory class (#7160)

langchain/memory/chat_message_histories/in_memory.py

@@ -9,6 +9,11 @@ from langchain.schema.messages import BaseMessage
 
 
 class ChatMessageHistory(BaseChatMessageHistory, BaseModel):
+    """In memory implementation of chat message history.
+
+    Stores messages in an in memory list.
+    """
+
     messages: List[BaseMessage] = []
 
     def add_message(self, message: BaseMessage) -> None: