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 the type operator is registered with `search_with` you should use that shared example instead.  NOTE: for the shared example you need to explicitly specify the `:name` even those the `search_with` call does not use it in the code under test because the shared examples finds the operator by name and won't ask the class under test for it's default operator name as that would allow the class under test to affect the test itself too much.

```ruby
it_should_behave_like 'search_with'Metasploit::Credential::Search::Operator::Type,
                      name: :type,
                      class_names: %w{
                        Metasploit::Credential::NonreplayableHash
                        Metasploit::Credential::NTLMHash
                        Metasploit::Credential::Password
                        Metasploit::Credential::SSHKey
                      }
```