Add help text and update specs.

[MSP-10733](https://jira.tor.rapid7.com/browse/MSP-10733)

# Verification

- [ ] `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


# Post-merge Steps

- [ ] remove the PRERELEASE constant from version.rb
- [ ] run

        $ rm Gemfile.lock
        $ bundle install
        $ rake release


I wasn't sure how to write a spec for :os here, since it is a derived operator