Add help text for Private and Public.

[MSP-10733](https://jira.tor.rapid7.com/browse/MSP-10733)

- [x] `$ cd spec/dummy; rails c`
- [ ] `Metasploit::Credential::Private.search_operator_by_name[:data].help` is correctly formatted and makes sense
- [ ] `Metasploit::Credential::Private.search_operator_by_name[:type].help` is correctly formatted and makes sense
- [ ] `Metasploit::Credential::Public.search_operator_by_name[:username].help` is correctly formatted and makes sense

# Post-merge Steps

Perform these steps prior to pushing to master.

- [ ] remove the PRERELEASE constant from version.rb
- [ ] run

        $ rm Gemfile.lock
        $ bundle install
        $ rake release

The example values you give are the `klass.model_name.human`, but the `klass.name` is also allowed, it's just longer.  I don't know if you want to include the `klass.name` in the list.