Make the formatting of private headers more consistent

This PR changes the documentation formatting on the private headers to be more consistent:

* Private headers now use two slashes for the docs, public headers use three.

* Private headers now either have "Private" in their file name or have "private" in the class or category doc.

The changes to the project file are because some headers were marked as project headers or public headers in the OS X static library target.

This could help alleviate the confusion amongst users about `RACDelegateProxy.h`.