Agora RTC Objective-C API Reference Refactor

### - b -

- background_source_type : VirtualBackgroundSource
- backgroundColor : LiveTranscoding , VideoCanvas , VideoCompositingLayout
- backgroundImage : LiveTranscoding
- backgroundImageCount : LiveTranscoding
- bandwidth_estimation_bps : DownlinkNetworkInfo
- bitrate : PublisherConfiguration , RtmpStreamingAudioConfiguration , RtmpStreamingVideoConfiguration , ScreenCaptureParameters , VideoEncoderConfiguration
- bitrate_action_counter : ILocalUser::ANAStats
- bitrateInKbps : SrcInfo
- blur_degree : VirtualBackgroundSource
- buffer : ExternalVideoFrame , IAudioFrameObserverBase::AudioFrame , ImagePayloadData , IMetadataObserver::Metadata , IPacketObserver::Packet
- buffered_pcm_data_list_size : ILocalAudioTrack::LocalAudioTrackStats
- burst_jitter : RemoteAudioTrackStats
- burst_peak_num : RemoteAudioTrackStats
- bytes_major_stream : LocalVideoTrackStats
- bytes_minor_stream : LocalVideoTrackStats
- bytes_per_sample : AudioPcmFrame
- bytes_sent : ILocalUser::LocalAudioDetailedStats
- bytesPerSample : IAudioFrameObserverBase::AudioFrame , RtmpStreamingAudioConfiguration