Add a YARD doc for private classes which aren't meant to be used externally.

[jeremyf]

Is there a consideration for using private_constant to ensure that programmatically ensure that constants aren't used externally?

[carolyncole]

Should use the private tag AND the flag on the yard doc creation. Should go right under the api tag description

[carolyncole]

Here is an example of the private_constant https://gist.github.com/zilkey/7109439