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.
>  this is used no where else than TD.

It will be used by TS as soon as someone implements the silo there.

`WithFire` is only used in RA because it was copied as part of the desert tileset.