Add parameter info for setSpeed

[ThioJoe]

I had a hunch that the unknown string parameter for the QETrackItem.setSpeed() method might take a timecode string, and I was right. It lets you explicitly set the duration of the clip on the timeline after setting the speed.

So I renamed the parameter from unknown to duration and added an example

I also added a description for the ripple parameter.

Example usage:

function main() {
    app.enableQE();
    var as = qe.project.getActiveSequence();
    var track0 = as.getVideoTrackAt(0);
    var clip = track0.getItemAt(0);
    clip.setSpeed(2.0, "00:00:22:30", false, true, false)
}