Add help text for Private and Public.

- [ ] `$ 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.

    $ rm Gemfile.lock
    $ bundle install
    $ rake release

You might want to explain what 'data' means for each type, like that it is the plain text password or hash or ssh private key.