Document `DiscordGameStatus` placeholder

`%online%` - [PresenceUpdater.java#L77](https://github.com/DiscordSRV/DiscordSRV/blob/6405e850eea61a4b9bf4f0d0cbb80fe20b3cfb90/src/main/java/github/scarsz/discordsrv/objects/threads/PresenceUpdater.java#L77)

Needs to be documented[ in `config.yml` as well](https://github.com/DiscordSRV/DiscordSRV/blob/6405e850eea61a4b9bf4f0d0cbb80fe20b3cfb90/src/main/resources/config/en.yml#L124-L135)