GstPlayer: Tell programmers that set_volume uses linear scale

gst_player_set_volume, and probably others, use a linear volume scale.

While this is not wrong per se, it would be nice to add a little comment in the documentation to alert programmers that a linear volume scale is not suitable for humans (a nice write-up on this: https://www.dr-lex.be/info-stuff/volumecontrols.html). For a widget with a volume slider for example, one should use a log scale.

I would have submitted a MR, but I don't know where to look so I thought this project wouldn't be too wrong to make an issue against.