code: public boolean isLinkable(TypeElement typeElem) {
          return
              (typeElem != null &&
                  (isIncluded(typeElem) && configuration.isGeneratedDoc(typeElem))) ||
              (configuration.extern.isExternal(typeElem) &&
                  (isPublic(typeElem) || isProtected(typeElem)));
      }
docs: Return true if this class is linkable and false if we can't link to the
  desired class.
  <br>
  <b>NOTE:</b>  You can only link to external classes if they are public or
  protected.
  
  @return true if this class is linkable and false if we can't link to the
  desired class.