Show Per-channel diarization section on Multichannel transcription page

[devin-ai-integration]

Summary

Uncomments the "Per-channel diarization" section on the Multichannel transcription documentation page, making it visible again. This reverses the change from PR #513 which previously hid this section.

The change removes the MDX comment syntax ({/* ... */}) that was wrapping the section.

Review & Testing Checklist for Human

• Verify the "Per-channel diarization" section appears on the Multichannel transcription page after deployment
• Confirm the section appears in the "On this page" sidebar navigation

Notes

Requested by: Lee Vaughn (leev@assemblyai.com)
Link to Devin run: https://app.devin.ai/sessions/add23edaea064ee294d46d0eb832d751

[devin-ai-integration]

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