docs(core): enhance docstring for RunnableConfig for clarity on total=False (#34756)

libs/core/langchain_core/runnables/config.py

@@ -51,8 +51,24 @@ class EmptyDict(TypedDict, total=False):
 class RunnableConfig(TypedDict, total=False):
     """Configuration for a `Runnable`.
 
-    See the [reference docs](https://reference.langchain.com/python/langchain_core/runnables/#langchain_core.runnables.RunnableConfig)
-    for more details.
+    !!! note Custom values
+
+        The `TypedDict` has `total=False` set intentionally to:
+
+        - Allow partial configs to be created and merged together via `merge_configs`
+        - Support config propagation from parent to child runnables via
+            `var_child_runnable_config` (a `ContextVar` that automatically passes
+            config down the call stack without explicit parameter passing), where
+            configs are merged rather than replaced
+
+        !!! example
+
+            ```python
+            # Parent sets tags
+            chain.invoke(input, config={"tags": ["parent"]})
+            # Child automatically inherits and can add:
+            # ensure_config({"tags": ["child"]}) -> {"tags": ["parent", "child"]}
+            ```
     """
 
     tags: list[str]
@@ -92,7 +108,8 @@ class RunnableConfig(TypedDict, total=False):
 
     configurable: dict[str, Any]
     """Runtime values for attributes previously made configurable on this `Runnable`,
-    or sub-Runnables, through `configurable_fields` or `configurable_alternatives`.
+    or sub-`Runnable` objects, through `configurable_fields` or
+    `configurable_alternatives`.
 
     Check `output_schema` for a description of the attributes that have been made
     configurable.