Agora RTC Objective-C API Reference Refactor

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- JOIN_CHANNEL_ERR_ALREADY_JOINED : agora::rtm
- JOIN_CHANNEL_ERR_EXCEED_LIMIT : agora::rtm
- JOIN_CHANNEL_ERR_FAILURE : agora::rtm
- JOIN_CHANNEL_ERR_INVALID_ARGUMENT : agora::rtm
- JOIN_CHANNEL_ERR_JOIN_SAME_CHANNEL_TOO_OFTEN : agora::rtm
- JOIN_CHANNEL_ERR_NOT_INITIALIZED : agora::rtm
- JOIN_CHANNEL_ERR_OK : agora::rtm
- JOIN_CHANNEL_ERR_REJECTED : agora::rtm
- JOIN_CHANNEL_ERR_TOO_OFTEN : agora::rtm
- JOIN_CHANNEL_ERR_USER_NOT_LOGGED_IN : agora::rtm
- JOIN_CHANNEL_TIMEOUT : agora::rtm