Chapter 4.8. Migrate the Threads Subsystem Configuration of the Migration Guide has a paragraph on "thread pools for connectors and listeners for the web subsystem" that are now configured by "referencing a worker that is defined in the io subsystem".
This text should have a link "for more info, see..." to chapter 16.13. Configuring the IO Subsystem in the Config Guide. (Note that there's a JIRA for moving this chapter one level up,
JBEAP-4081, but that shouldn't affect this at all.)