Optimize restream docs

[NickM-27]

It's better to use

go2rtc:
  streams:
    test_cam:
      - rtsp://192.168.1.5:554/live0
      - ffmpeg:test_cam#audio=aac  # or audio=opus

Because:

1. Start will be faster. FFmpeg has high start time for RTSP with video. Usual: 2 * keyframe time.
2. Go2rtc may not run ffmpeg if first source already has suitable video+audio
   Start ffmpeg with only audio will be fast.

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[felipecrs]

Actually this optimization is quite dramatic. It drops multiple times the amount of CPU used by go2rtc.