core[runnables]: docstring for class RunnableSerializable, method configurable_fields (#19722)

**Description:** Update to the docstring for class RunnableSerializable,
method configurable_fields
**Issue:** [Add in code documentation to core Runnable methods
#18804](https://github.com/langchain-ai/langchain/issues/18804)
**Dependencies:** None

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/core/langchain_core/runnables/base.py

@@ -2034,6 +2034,33 @@ class RunnableSerializable(Serializable, Runnable[Input, Output]):
     def configurable_fields(
         self, **kwargs: AnyConfigurableField
     ) -> RunnableSerializable[Input, Output]:
+        """Configure particular runnable fields at runtime.
+
+        .. code-block:: python
+
+            from langchain_core.runnables import ConfigurableField
+            from langchain_openai import ChatOpenAI
+
+            model = ChatOpenAI(max_tokens=20).configurable_fields(
+                max_tokens=ConfigurableField(
+                    id="output_token_number",
+                    name="Max tokens in the output",
+                    description="The maximum number of tokens in the output",
+                )
+            )
+
+            # max_tokens = 20
+            print(
+                "max_tokens_20: ",
+                model.invoke("tell me something about chess").content
+            )
+
+            # max_tokens = 200
+            print("max_tokens_200: ", model.with_config(
+                configurable={"output_token_number": 200}
+                ).invoke("tell me something about chess").content
+            )
+        """
         from langchain_core.runnables.configurable import RunnableConfigurableFields
 
         for key in kwargs: