Update 02-how-to.md

[jonathonherbert]

What does this change?

Updates our migration docs based on feedback using a stooge (moi) to kick off a migratin.

How should a reviewer test this change?

Do the changes read well? Do they add value?

[andrew-nowak]

• [x] ~todo: doc thrall.projection.parallelism param~ done

[prout-bot]

Seen on auth, usage, cropper, collections, media-api, kahuna, image-loader, metadata-editor, thrall, leases (created by @jonathonherbert and merged by @andrew-nowak 10 minutes and 50 seconds ago) Please check your changes!