message: doc: improve listbanned help
  Add descriptions for the address, ban_created, and banned_until fields.
diff: @@ -749,9 +749,9 @@ static RPCHelpMan listbanned()
  {
  {RPCResult::Type::OBJ, "", "",
  {
  - {RPCResult::Type::STR, "address", ""},
  - {RPCResult::Type::NUM_TIME, "ban_created", ""},
  - {RPCResult::Type::NUM_TIME, "banned_until", ""},
  + {RPCResult::Type::STR, "address", "The IP/Subnet of the banned node"},
  + {RPCResult::Type::NUM_TIME, "ban_created", "The " + UNIX_EPOCH_TIME + " the ban was created"},
  + {RPCResult::Type::NUM_TIME, "banned_until", "The " + UNIX_EPOCH_TIME + " the ban expires"},
  }},
  }},
  RPCExamples{
  