improve documentation

fixes #1391

and add improvements/fixes to documentation
Yes, it' a poor-man's simple animation. This is made on purpose as is quickstart example. Performance is completely unconsidered here. All I wanted is provide a simple line animation that is easy to understand for beginners. (task in which I'm probably failing ;-) )

The first loop changes the y values of the line and the second one revert it back to initial situation.

I'd be happy to replace this with suggestions that are more straightforward/easier to understand.