code: public static boolean isLinkable(ClassDoc classDoc,
              Configuration configuration) {
          return
              ((classDoc.isIncluded() && configuration.isGeneratedDoc(classDoc))) ||
              (configuration.extern.isExternal(classDoc) &&
                  (classDoc.isPublic() || classDoc.isProtected()));
      }
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.
  
  @param classDoc the class to check.
  @param configuration the current configuration of the doclet.
  
  @return true if this class is linkable and false if we can't link to the
  desired class.