commands: correct documentation of hg serve’s --ipv6 option
When the --ipv6 option is given, the server doesn’t listen to a IPv4 socket.
This can be verified by running two servers, one with and one
without the option, which works fine.
I think that listening to both a IPv4 and a IPv6 socket would be better,
but given that the Python standard library class underlying the
HTTP server supports only one socket, this is not trivial.
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