8to9 guide anti-documents removed re-routing feature

[pinko-fowle]

• Used appropriate template for the issue type
• Searched both open and closed issues for duplicates of this issue
• Title adequately and concisely reflects the feature or the bug

Restify Version: 9
Node.js Version: 16

Expected behaviour

The 8to9 guide should provide help & assistance for people affected by #1847, the removal of re-routing.

Actual behaviour

The 8to9 guide falsly says re-routing works in 9.0 async handlers. And implies it is still available for classic handlers.

Repro case

Cause

Are you willing and able to fix this?

If I knew what the go-forward path was & how to migrate, I'd gladly submit a PR to update 8to9 guide.