doc(set_theory/game/nim): update module docs (#15406)
doc(set_theory/game/nim): update module docs (#15406)

We update the module docs to reflect that you should use `to_left_moves_nim` and `to_right_moves_nim` to interface with the nim API, instead of using the raw definition (which has the problems the old docs mentioned).





leanprover-community/mathlib,leanprover-community/mathlib