Correct the offload capability and update comment

- Add some comments to describe the supported scenario
- Correct the config from mono to stereo, when only one device is
  connected, mono only need one audio channel, for stereo case,
  support 2 audio channels.

Tag: #feature
Bug: 203535499
Bug: 150670922
Test: make build
Change-Id: I6dacca8965f48ed58506b63f8d3939afb6f5b1b6