docs(core): update StrOutputParser docstring (#34213)

Author: mdrxy

libs/core/langchain_core/output_parsers/string.py

@@ -6,7 +6,33 @@
 
 
 class StrOutputParser(BaseTransformOutputParser[str]):
-    """OutputParser that parses `LLMResult` into the top likely string."""
+    """Extract text content from model outputs as a string.
+
+    Converts model outputs (such as `AIMessage` or `AIMessageChunk` objects) into plain
+    text strings. It's the simplest output parser and is useful when you need string
+    responses for downstream processing, display, or storage.
+
+    Supports streaming, yielding text chunks as they're generated by the model.
+
+    Example:
+        ```python
+        from langchain_core.output_parsers import StrOutputParser
+        from langchain_openai import ChatOpenAI
+
+        model = ChatOpenAI(model="gpt-4o")
+        parser = StrOutputParser()
+
+        # Get string output from a model
+        message = model.invoke("Tell me a joke")
+        result = parser.invoke(message)
+        print(result)  # plain string
+
+        # With streaming - use transform() to process a stream
+        stream = model.stream("Tell me a story")
+        for chunk in parser.transform(stream):
+            print(chunk, end="", flush=True)
+        ```
+    """
 
     @classmethod
     def is_lc_serializable(cls) -> bool:

libs/core/langchain_core/runnables/base.py

@@ -127,10 +127,10 @@ class Runnable(ABC, Generic[Input, Output]):
     Key Methods
     ===========
 
-    - **`invoke`/`ainvoke`**: Transforms a single input into an output.
-    - **`batch`/`abatch`**: Efficiently transforms multiple inputs into outputs.
-    - **`stream`/`astream`**: Streams output from a single input as it's produced.
-    - **`astream_log`**: Streams output and selected intermediate results from an
+    - `invoke`/`ainvoke`: Transforms a single input into an output.
+    - `batch`/`abatch`: Efficiently transforms multiple inputs into outputs.
+    - `stream`/`astream`: Streams output from a single input as it's produced.
+    - `astream_log`: Streams output and selected intermediate results from an
         input.
 
     Built-in optimizations: