Add help text and update specs.

- [ ] `cd spec/dummy; rails c`
- [ ] `Mdm::Service.search_operator_by_name[:info].help` looks correct
- [ ] `Mdm::Service.search_operator_by_name[:name].help` looks correct
- [ ] `Mdm::Service.search_operator_by_name[:proto].help` looks correct
- [ ] `Mdm::Service.search_operator_by_name[:port].help` looks correct
- [ ] `Mdm::Host.search_operator_by_name[:name].help` looks correct
- [ ] `Mdm::Host.search_operator_by_name[:address].help` looks correct
- [ ] `Mdm::Host.search_operator_by_name[:os_name].help` looks correct
- [ ] `Mdm::Host.search_operator_by_name[:os_flavor].help` looks correct
- [ ] `Mdm::Tag.search_operator_by_name[:name].help` looks correct
- [ ] `Mdm::Tag.search_operator_by_name[:desc].help` looks correct