chore(api): clarify Realtime modality array definitions (#3954)

## Summary

Clarifies retained Realtime modality arrays in the API reference without
changing SDK types or method signatures.

## Changes

- Explicitly describe retained Realtime modality fields as arrays of
text or audio values.
- Preserve existing optionality and SDK behavior.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>