docs: Improve the description of build-game1-start.js

[dckc]

We're trying to get away from calling this a "game" (#18, #22). So I hesitate to dig the hole deeper.

In #38, I scrubbed "game" from many places, but I think I broke a bunch of stuff in the process. I aim to do it again, more carefully.

[dckc]

The rename and such (#45) has landed. Are you interested to pick this up again?

49 added some docs... but only in the contract, I suppose.

Perhaps I should pick this up when I do...

• https://github.com/Agoric/documentation/issues/917

[gibson042]

@dckc Done and ready for review.