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 staging/electro-release or the build will be broke on staging/electro-release.

## Version
- [ ] Edit `lib/metasploit/credential/version.rb`
- [ ] Change `PRELEASE` to `electro-release` to match the branch (staging/electro-release) relative name (electro-release)

## Gem build
- [ ] gem build *.gemspec
- [ ] VERIFY the pre release name has change to `pre.electro.pre.release` on the gem.

## RSpec
- [ ] `rake spec`
- [ ] VERIFY version examples pass without failures

## Commit & Push
- [ ] Commit and push changed to `lib/metasploit/credential/version.rb` to staging/electro-release.