Migration Guide: missing content on migrating the "web" subsystem's usage of the "threads" subsystem

As mentioned in JBEAP-3915, the chapter 4.7. Migrate the Threads Subsystem Configuration should have a bit of text to explain how to migrate the web subsystem's usage of the threads subsystem.

In other words: in EAP 6, the infinispan, jgroups and web subsystems could use the threads subsystem to configure thread pools. The Migration Guide already covers migration of this for the infinispan and jgroups subsystems. The web one is missing. There's a :migrate operation for the web subsystem, but that can't migrate the references to the threads subsystem, so the user has to do that manually. Hence, inclusion into chapter 4.7. Migrate the Threads Subsystem Configuration is justified.

I think it doesn't have to be long. Just note that where you referenced executors from the threads subsystem in EAP 6, you now reference workers from the io subsystem in EAP 7.

CC rhatlapa.