code: @SuppressWarnings({"unchecked"})
      public @Nonnull <T extends Describable<T>,D extends Descriptor<T>> DescriptorExtensionList<T,D> getDescriptorList(Class<T> type) {
          return descriptorLists.computeIfAbsent(type, key -> DescriptorExtensionList.createDescriptorList(this, key));
      }
docs: Returns {@link ExtensionList} that retains the discovered {@link Descriptor} instances for the given
  kind of {@link Describable}.
  
  @return
  Can be an empty list but never null.