Queue-ordered (`+queue1,queue2`, specifically the `+`) has performance problems

[bensheldon]

Using the + when defining GOOD_JOB_QUEUES seems to have some significant performance problems, as uncovered here: https://github.com/bensheldon/good_job/discussions/1091

This feature was introduced here: https://github.com/bensheldon/good_job/pull/665

...and I don't think we did any index tuning for it.

As a start, it might be good just to add a note in the README that it's usable, but not particularly recommended

(Aside: not infrequently, I think there is a perception that using all the features is the ideal situation, or that features listed at the bottom are "advanced" when sometimes they come last because they're not important or not particularly recommended... and good docs should be explicit in those situations. I'll try to remember that 😓 )