Clarify SyncParallelPipeline docstrings

Rewrite docstrings to more clearly explain what SyncParallelPipeline
does: hold all output until every parallel branch finishes, so frames
produced in response to a single input are released together.

src/pipecat/pipeline/sync_parallel_pipeline.py

@@ -4,11 +4,16 @@
 # SPDX-License-Identifier: BSD 2-Clause License
 #
 
-"""Synchronous parallel pipeline implementation for concurrent frame processing.
+"""Synchronized parallel pipeline that holds output until all branches finish.
 
-This module provides a pipeline that processes frames through multiple parallel
-pipelines simultaneously, synchronizing their output to maintain frame ordering
-and prevent duplicate processing.
+A SyncParallelPipeline fans each inbound frame out to multiple parallel pipelines
+and waits for every pipeline to finish processing before releasing any of the
+resulting output frames. This ensures that all frames produced in response to a
+single input frame are emitted together.
+
+System frames (except EndFrame) are exempt from this synchronization — they pass
+straight through without waiting, since they are expected to race ahead of
+regular data frames.
 """
 
 import asyncio
@@ -46,20 +51,21 @@ class FrameOrder(Enum):
 
 @dataclass
 class SyncFrame(ControlFrame):
-    """Control frame used to synchronize parallel pipeline processing.
+    """Sentinel frame used to detect when a parallel pipeline has finished processing.
 
-    This frame is sent through parallel pipelines to determine when the
-    internal pipelines have finished processing a batch of frames.
+    After sending a real frame into a parallel pipeline, a SyncFrame is sent
+    behind it. When the SyncFrame emerges from the pipeline's output, we know
+    all output frames for the preceding input have been produced.
     """
 
     pass
 
 
 class SyncParallelPipelineSource(FrameProcessor):
-    """Source processor for synchronous parallel pipeline processing.
+    """Bookend processor placed at the start of each parallel pipeline.
 
-    Routes frames to parallel pipelines and collects upstream responses
-    for synchronization purposes.
+    Forwards downstream frames into the pipeline and captures upstream frames
+    into a queue so the parent SyncParallelPipeline can release them later.
     """
 
     def __init__(self, upstream_queue: asyncio.Queue):
@@ -88,10 +94,11 @@ class SyncParallelPipelineSource(FrameProcessor):
 
 
 class SyncParallelPipelineSink(FrameProcessor):
-    """Sink processor for synchronous parallel pipeline processing.
+    """Bookend processor placed at the end of each parallel pipeline.
 
-    Collects downstream frames from parallel pipelines and routes
-    upstream frames back through the pipeline.
+    Captures downstream output frames into a queue so the parent
+    SyncParallelPipeline can release them later, and forwards upstream
+    frames back through the pipeline.
     """
 
     def __init__(self, downstream_queue: asyncio.Queue):
@@ -120,15 +127,20 @@ class SyncParallelPipelineSink(FrameProcessor):
 
 
 class SyncParallelPipeline(BasePipeline):
-    """Pipeline that processes frames through multiple parallel pipelines synchronously.
+    """Fans each input frame to parallel pipelines then holds output until every pipeline finishes.
 
-    Creates multiple parallel processing paths that all receive the same input frames
-    and produces synchronized output. Each parallel path is a separate pipeline that
-    processes frames independently, with synchronization points to ensure consistent
-    ordering and prevent duplicate frame processing.
+    For each inbound frame the pipeline:
 
-    The pipeline uses SyncFrame control frames to coordinate between parallel paths
-    and ensure all paths have completed processing before moving to the next frame.
+    1. Sends the frame into every parallel pipeline.
+    2. Sends a ``SyncFrame`` sentinel behind it in each pipeline.
+    3. Waits until every pipeline has produced its ``SyncFrame``, meaning all
+       output for that input is ready.
+    4. Releases the collected output frames (deduplicating by frame id, since
+       the same frame may emerge from more than one branch).
+
+    System frames (except ``EndFrame``) bypass this mechanism entirely — they are
+    forwarded through each pipeline and pushed immediately, since system frames
+    are expected to race ahead of regular data frames.
 
     By default, output frames are pushed in the order they arrive from any pipeline
     (``FrameOrder.ARRIVAL``). Set ``frame_order=FrameOrder.PIPELINE`` to push frames
@@ -239,16 +251,11 @@ class SyncParallelPipeline(BasePipeline):
         await asyncio.gather(*[p.cleanup() for p in self._pipelines])
 
     async def process_frame(self, frame: Frame, direction: FrameDirection):
-        """Process frames through all parallel pipelines with synchronization.
+        """Send a frame through all parallel pipelines and release output once all finish.
 
-        Distributes frames to all parallel pipelines and synchronizes their output
-        to maintain proper ordering and prevent duplicate processing. Uses SyncFrame
-        control frames to coordinate between parallel paths.
-
-        When ``frame_order`` is ``FrameOrder.ARRIVAL``, output frames are pushed in
-        the order they arrive from any pipeline (via a shared queue). When it is
-        ``FrameOrder.PIPELINE``, each pipeline collects its output into a separate
-        list and the lists are drained in pipeline definition order.
+        System frames (except EndFrame) skip synchronization and pass straight
+        through. All other frames are fanned out to every pipeline, and output is
+        held until every pipeline signals completion (via SyncFrame).
 
         Args:
             frame: The frame to process.