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

Since this search needs to work outside of metasploit-credential itself, I would recommend use the terminology "private credentials" instead of just "privates".  I've tried to do this in the association naming and YARD documentation too.