Update addon registry documentation

[dfreeman]

Per #439, we want to update our documentation for how addons should expose their template registry to consumers to avoid having anyone but the host application do actual declaration merging.

The two pages I'm aware of that need to be updated are:

• https://typed-ember.gitbook.io/glint/using-glint/ember/using-addons
• https://typed-ember.gitbook.io/glint/using-glint/ember/authoring-addons

We could also add an entry to the Migrating page, though my leaning is that we should just leave that to addon authors to note in their releases instead.