Add hint about replication slots when nearing wraparound

Hi,

While doing some wraparound debugging, I saw the hint regarding upcoming
wraparound did not include the problem of having a stale replication
slot (which I'm actually using to force wraparound issues).

I remember a few discussions where a stale replication slot was actually
the culprit in these situations.

Something like the attached maybe?

regards,

Feike