We added the built in datatypes to the table of contents for Schemas part 2 because we realized that users of the specification frequently want to look up information about them.  I think the same applies to construction by list (section 2.4.1.2) and Union (2.4.1.3).  The fact that they are introduced in level 4 sections complicates making a specific proposal for how to do this, but I think users would value the ability to easily find information on how list and union types work.