Use `stable` variable name instead `nonStable`

Having the variable be `nonStable` instead of `stable` is a bit unintuitive. I think the fact that you need to add this clarification to the function comment here is evidence for this -- you need to be very careful to explain what the parameter does, when if it were just `stable bool`, then it would be obvious.

_Originally posted by @rickystewart in https://github.com/cockroachdb/cockroach-go/pull/184#discussion_r2164482171_