resolv.conf.5 says: nameserver Name server IP address Internet address (in dot notation) of a name server that the resolver should query. Only IPv4 addresses have a "dot notation", so this suggests that IPv6 addresses aren't allowed. But they are. Given that the "sortlist" field is IPv4-only, it's probably best to actually be explicit that nameserver supports either. So: nameserver Name server IP address IP address (IPv4 or IPv6) of a name server that the resolver should query.
As of man-pages-3.51 resolv.conf.5: nameserver Name server IP address Internet address of a name server that the resolver should query, either an IPv4 address (in dot notation), or an IPv6 address in colon (and possibly dot) notation as per RFC 2373. So this bug can be closed.