Add help text and update specs.

[MSP-10733](https://jira.tor.rapid7.com/browse/MSP-10733)

# Verification

- [x] `cd spec/dummy; rails c`
- [x] `Mdm::Service.search_operator_by_name[:info].help` looks correct
- [x] `Mdm::Service.search_operator_by_name[:name].help` looks correct
- [x] `Mdm::Service.search_operator_by_name[:proto].help` looks correct
- [x] `Mdm::Service.search_operator_by_name[:port].help` looks correct
- [x] `Mdm::Host.search_operator_by_name[:name].help` looks correct
- [x] `Mdm::Host.search_operator_by_name[:address].help` looks correct
- [x] `Mdm::Host.search_operator_by_name[:os_name].help` looks correct
- [x] `Mdm::Host.search_operator_by_name[:os].help` looks correct
- [x] `Mdm::Tag.search_operator_by_name[:name].help` looks correct
- [x] `Mdm::Tag.search_operator_by_name[:desc].help` looks correct


# Post-merge Steps

- [ ] remove the PRERELEASE constant from version.rb
- [ ] run

        $ rm Gemfile.lock
        $ bundle install
        $ rake release