rune: animate_rune not enough documentation

[martin-henz]

Documentation of func parameter too vague. Currently:

func: RuneAnimation
Takes in the timestamp and returns a Rune to draw

Should say: The parameter n refers to the time (in seconds) at which the rune should be shown. So in David's example, anim will be called 601 times, with arguments 0, ..., 10, in step sizes of 1/60. The duration of the animation is 10 seconds and the display rate ist 60 frames per second.