The set_list_stream and get_list_stream method descriptions in AudioStreamPlaylist should be clarified

Godot version 4.3

The current descriptions for the `set_list_stream` and `get_list_stream` methods in the [AudioStreamPlaylist](https://docs.godotengine.org/en/4.3/classes/class_audiostreamplaylist.html) documentation are misleading. The descriptions currently reference the "playback position index", but the AudioStreamPlaylist does not deal with the playback position directly. I am proposing removing the "playback position" wording and just having the description read "the specified index".

I went ahead and made a PR ([#95877](https://github.com/godotengine/godot/pull/95877)) for this change as it is relatively minor, but if anyone has better wording for these two method descriptions, please share and I can update the PR.