doc,test: fix prime generation description

The previous description incorrectly explained the behavior of
options.add and options.rem for primes that are not safe.

PR-URL: https://github.com/nodejs/node/pull/37085


Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>