chore(api): clarify incoming SIP call ID usage (#3885)

## Summary

Clarifies how to use incoming SIP call identifiers with the Realtime and
Live APIs. This update changes documentation only.

## Changes

- Document that incoming call IDs should be passed unchanged to Realtime
accept and reject requests.
- Direct Live API callers to the separate session ID from the incoming
transport event.

Co-authored-by: jbeckwith-oai <269036200+jbeckwith-oai@users.noreply.github.com>