Use `stable` variable name instead `nonStable`

[rail]

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