docs: FlameGame<W extends World> needs documentation+dartdocs about `W`

[spydon]

The generics of FlameGame needs a sentence or two in the Flame docs and as dartdocs.

Originally posted by @IanWadham in https://github.com/flame-engine/flame/pull/2842#discussion_r1397089423

[Rajkdea123]

Can I work on this issue sir?

[spydon]

Go for it, I'll assign you to the issue