From tor-relays mailing list:
"If you enable IPv6 without working IPv6 connectivity your entire relay will not be used, regardless if IPv4 is working."
This seems to me to be quite idiosyncratic to tor, and easily forgotten - not the first thing I would think about when troubleshooting, nor is the post-install page quite where I would look for help when a relay stopped. As we move more and more to including IPv6 in our relay setups, this will come up more often. But we should at least add bold here.