Documented the various overlay and animation change traits

They all kinda sound the same, are often named very generic and used hard-coded sequence names that forced people to read the source code or wait for the game to crash to find out. This should improve https://github.com/OpenRA/OpenRA/wiki/Traits a lot in that regard. Includes some minor cleanup as well.
I don't see a strong use-case for moving this (especially not in a patch that claims to only add documentation!) - this is not TD-specific like the gunboat or oil derrick fire, and so it may be useful for other mods.