audiodecoder and audioencoder: Fix get_audio_info return ownership GTK-Doc

GTK-Doc specifies that, by default, the caller owns returned objects, so that the caller should free them when it is done. However, in the case of this function, the returned GstAudioInfo is owned by the decoder, so this default choice is incorrect. This creates double free problems when using GStreamer Rust bindings, because they are generated using the information contained in the docs.

Fix this by correctly specifying that the caller does not own the returned object.