docs: improved migration guide for onRoute

[ShogunPanda]

This PR improves the migration guide to explicit how to define routes when onRoute is also used.

[climba03003]

Why are you think it is misleading?
The solution can be chosen from either one, and now it is totally different meaning.

[ShogunPanda]

Ehm, because I tried. :)
I noticed while upgrading my npm package fastify-print-routes to fastify v4.
Using fastify-plugin is recommended but it is not enough to make that working, await register is the key.

[climba03003]

wrap your routes in a plugin (recommended)

import Fastify from 'fastify'
import FP from 'fastify-plugin'

const fastify = Fastify()
fastify.register(FP(function(fastify, option, done) {
  fastify.addHook('onRoute', function() {
    console.log('onRoute')
  })
  done()
}))

fastify.register(function(fastify, option, done) {
  fastify.get('/', () => 'hello')
  done()
})

use await register(...)

import Fastify from 'fastify'
import FP from 'fastify-plugin'

const fastify = Fastify()
await fastify.register(FP(function(fastify, option, done) {
  fastify.addHook('onRoute', function() {
    console.log('onRoute')
  })
  done()
}))

fastify.get('/', () => 'hello')

[ShogunPanda]

Ops. It makes sense now. I'll change it so it's more explicit.

PR contents and title updated.

[mcollina]

lgtm

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