docs: add warning that speaker_options apply per-channel with multichannel transcription

[devin-ai-integration]

Summary

Adds <Warning> callouts to both the Multichannel Transcription and Speaker Diarization docs pages noting that speaker_options (min_speakers_expected / max_speakers_expected) are applied per channel, not globally, when multichannel: true and speaker_labels: true are used together.

This was flagged internally as a major customer gotcha — e.g. setting min=5, max=7 on a 5-channel file results in 25–35 total speakers, not 5–7.

Files changed:

• fern/pages/speech-to-text/pre-recorded-audio/multichannel-transcription.mdx — warning added in the "Per-channel diarization" section
• fern/pages/speech-to-text/pre-recorded-audio/speaker-diarization.mdx — warning added in the "Set a range of possible speakers" section

Review & Testing Checklist for Human

• Verify the described behavior is accurate — confirm that speaker_options are indeed applied per-channel (not globally) when multichannel is enabled. This claim comes from an internal investigation but should be validated by someone familiar with the diarization backend.
• Preview the rendered docs to confirm the <Warning> blocks render correctly and the cross-link from the speaker diarization page to the multichannel page resolves properly (/docs/speech-to-text/pre-recorded-audio/multichannel-transcription).

Notes

• The warning text is intentionally similar on both pages since users may land on either one independently.

Link to Devin session: https://app.devin.ai/sessions/64d717aba1a74942a8d222b1d187fb3e
Requested by: @ryanseams

[devin-ai-integration]

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