The DocumentationComment class was moved out of the compiler layer and made internal because it was determined to be insufficient at describing all the details in a documentation comment, since it contained other fragments like see and cref, and could in fact contain any other xml tag you wished. The only structure that made sense as part of the public API was a string of xml from which you could use a xml reader or DOM to pick it apart. As it is now, the DocumentationComment class is an implementation detail of a single Roslyn feature.
Since it is open source you can use it if you wish by copying the code. The actual internal class is subject to change and could even be removed