extend_info_describe should list IPv6 address (if present)

When running an IPv6-only client using:

src/or/tor ClientUseIPv4 0 DataDirectory `mktemp -d` UseMicrodescriptors 0

I get log messages like [info] add_an_entry_guard(): Chose $906933A309F15D7CF379127078A6791DF9B0C763~Unnamed at 91.121.82.25 as new entry guard

But I'm really contacting them on their IPv6 address. So we should list it along with the IPv4 address.

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