[Doc] Improve api doc (#30073)

- Update api_doc for `BaseMessage`
- add static method decorator for `retry_runnable`

libs/core/langchain_core/messages/base.py

@@ -145,11 +145,11 @@ def merge_content(
     first_content: Union[str, list[Union[str, dict]]],
     *contents: Union[str, list[Union[str, dict]]],
 ) -> Union[str, list[Union[str, dict]]]:
-    """Merge two message contents.
+    """Merge multiple message contents.
 
     Args:
         first_content: The first content. Can be a string or a list.
-        second_content: The second content. Can be a string or a list.
+        contents: The other contents. Can be a string or a list.
 
     Returns:
         The merged content.

libs/core/langchain_core/runnables/retry.py

@@ -135,8 +135,8 @@ class RunnableRetry(RunnableBindingBase[Input, Output]):
     def _async_retrying(self, **kwargs: Any) -> AsyncRetrying:
         return AsyncRetrying(**self._kwargs_retrying, **kwargs)
 
+    @staticmethod
     def _patch_config(
-        self,
         config: RunnableConfig,
         run_manager: "T",
         retry_state: RetryCallState,