The set_list_stream and get_list_stream method descriptions in AudioStreamPlaylist should be clarified

[Braxton901]

Godot version 4.3

The current descriptions for the set_list_stream and get_list_stream methods in the AudioStreamPlaylist 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) 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.