Migration Guide: the text on migrating from "threads" subsystem to "io" subsystem for the old "web" subsystem should link to the Config Guide

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.)