docs(assemblyai): add vad_threshold parameter documentation

Add documentation for the vad_threshold parameter in AssemblyAI U3 Pro:

- Add vad_threshold to AssemblyAIConnectionParams table
- Add usage example showing VAD threshold alignment with Silero VAD
- Add note about VAD threshold alignment to avoid "dead zone"
- Explain the misalignment issue between AssemblyAI (default 0.3) and
  Pipecat's Silero VAD (default 0.7)

This corresponds to the vad_threshold parameter added in pipecat-ai/pipecat#3927

Author: zkleb-aai

server/services/stt/assemblyai.mdx

@@ -137,6 +137,7 @@ Connection-level parameters passed via the `connection_params` constructor argum
 | `language_detection`                     | `bool`      | `None`         | Enable automatic language detection. Only applicable to `universal-streaming-multilingual`. Turn messages include language information. |
 | `format_turns`                           | `bool`      | `True`         | Whether to format transcript turns.                                                                                                     |
 | `speaker_labels`                         | `bool`      | `None`         | Enable speaker diarization. Final transcripts include a speaker field (e.g., "Speaker A", "Speaker B").                                 |
+| `vad_threshold`                          | `float`     | `None`         | Voice activity detection confidence threshold. Only applicable to `u3-rt-pro`. The confidence threshold (0.0 to 1.0) for classifying audio frames as silence. Frames with VAD confidence below this value are considered silent. Increase for noisy environments to reduce false speech detection. Defaults to 0.3 (API default). For best performance when using with external VAD (e.g., Silero), align this value with your VAD's activation threshold. Defaults to `None` (not sent). |
 
 ## Usage
 
@@ -206,9 +207,33 @@ stt = AssemblyAISTTService(
 )
 ```
 
+### With Aligned VAD Thresholds
+
+When using AssemblyAI with an external VAD (e.g., Silero VAD), align both thresholds to avoid a "dead zone" where AssemblyAI transcribes speech that your VAD hasn't detected yet:
+
+```python
+from pipecat.services.assemblyai import AssemblyAISTTService
+from pipecat.services.assemblyai.models import AssemblyAIConnectionParams
+from pipecat.audio.vad.silero import SileroVADAnalyzer, VADParams
+
+# Align both thresholds to 0.3 for optimal performance
+stt = AssemblyAISTTService(
+    api_key=os.getenv("ASSEMBLYAI_API_KEY"),
+    connection_params=AssemblyAIConnectionParams(
+        speech_model="u3-rt-pro",
+        vad_threshold=0.3,  # Match with Silero VAD
+    ),
+)
+
+vad = SileroVADAnalyzer(
+    params=VADParams(confidence=0.3)  # Match with AssemblyAI
+)
+```
+
 ## Notes
 
-- **u3-rt-pro model**: The default model is now `u3-rt-pro`, which provides the best performance and supports built-in turn detection. 
+- **u3-rt-pro model**: The default model is now `u3-rt-pro`, which provides the best performance and supports built-in turn detection.
+- **VAD threshold alignment**: When using AssemblyAI with external VAD systems (e.g., Silero VAD), align the `vad_threshold` parameter with your VAD's activation threshold (recommended: 0.3 for both) to avoid a "dead zone" where AssemblyAI transcribes speech that your VAD hasn't detected yet. This misalignment can delay interruption handling. AssemblyAI's `vad_threshold` defaults to 0.3, while Pipecat's Silero VAD defaults to 0.7 - we recommend lowering Silero to 0.3 to match.
 - **Turn detection modes**:
   - **Pipecat mode** (`vad_force_turn_endpoint=True`, default): Forces AssemblyAI to return finals ASAP so Pipecat's turn detection (e.g., Smart Turn) decides when the user is done. The service sends a `ForceEndpoint` message when VAD detects the user has stopped speaking.
   - **AssemblyAI mode** (`vad_force_turn_endpoint=False`, u3-rt-pro only): AssemblyAI's model controls turn endings using built-in turn detection. The service emits `UserStartedSpeakingFrame` and `UserStoppedSpeakingFrame` based on AssemblyAI's detection.