message: net: document `enum Network`
diff: #include <string>
  #include <vector>
  +/**
  + * A network type.
  + * @note An address may belong to more than one network, for example `10.0.0.1`
  + * belongs to both `NET_UNROUTABLE` and `NET_IPV4`.
  + * Keep these sequential starting from 0 and `NET_MAX` as the last entry.
  + * We have loops like `for (int i = 0; i < NET_MAX; i++)` that expect to iterate
  + * over all enum values and also `GetExtNetwork()` "extends" this enum by
  + * introducing standalone constants starting from `NET_MAX`.
  + */
  enum Network
  {
  + /// Addresses from these networks are not publicly routable on the global Internet.
  NET_UNROUTABLE = 0,
  +
  + /// IPv4
  NET_IPV4,
  +
  + /// IPv6
  NET_IPV6,
  +
  + /// TORv2
  NET_ONION,
  +
  + /// A set of dummy addresses that map a name to an IPv6 address. These
  + /// addresses belong to RFC4193's fc00::/7 subnet (unique-local addresses).
  + /// We use them to map a string or FQDN to an IPv6 address in CAddrMan to
  + /// keep track of which DNS seeds were used.
  NET_INTERNAL,
  + /// Dummy value to indicate the number of NET_* constants.
  NET_MAX,
  };
  