You're seeing just the function update_voice_state, go back to Nostrum.Api module for more information.
Link to this function

update_voice_state(guild_id, channel_id, self_mute \\ false, self_deaf \\ false)

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update_voice_state(, | nil,
) :: no_return() | :ok

Joins, moves, or disconnects the bot from a voice channel.

The correct shard to send the update to will be inferred from the guild_id. If a corresponding guild_id is not found a cache error will be raised.

To disconnect from a channel, channel_id should be set to nil.