docs: provide correct, non misleading usage of the `onStart` callback. Also clarify a bit the `onStop` paragraph.
docs: provide correct, non misleading usage of the `onStart` callback. Also clarify a bit the `onStop` paragraph.

EskenderDev/tween.js,CasualBot/tween.js,EskenderDev/tween.js,CasualBot/tween.js,altereagle/tween.js,altereagle/tween.js,camellhf/tween.js,camellhf/tween.js,CasualBot/tween.js,camellhf/tween.js,altereagle/tween.js,EskenderDev/tween.js