Potion Javadoc improvements

 - Document fromDamage() and fromItemStack().
 - Remove those needless & stupid linebreaks
 - Add @deprecated Javadoc to Tier (so that an explanation shows up).
 - Improve the class description

See also: https://github.com/riking/Bukkit/commits/extendpotion-throw
Those JDoc improvements require a code change, and will cause compile warnings. I need to check whether they're ready for a PR still.