Module documentation for langchain runnables (#11550)

Add in code documentation for langchain runnables module.

libs/langchain/langchain/schema/runnable/__init__.py

@@ -1,3 +1,19 @@
+"""LangChain Runnables and the LangChain Expression Language (LCEL).
+
+The LangChain Expression Language (LCEL) offers a declarative method to build
+production-grade programs that harness the power of LLMs.
+
+Programs created using LCEL and LangChain Runnables inherently support
+synchronous, asynchronous, batch, and streaming operations.
+
+Support for async allows servers hosting LCEL based programs to scale better
+for higher concurrent loads.
+
+Streaming of intermediate outputs as they're being generated allows for
+creating more responsive UX.
+
+This module contains schema and implementation of LangChain Runnables primitives.
+"""
 from langchain.schema.runnable._locals import GetLocalVar, PutLocalVar
 from langchain.schema.runnable.base import (
     Runnable,