`readonly` has been marked private since 2016

https://github.com/emberjs/ember.js/blob/452b9010defb5e4eef3d9612aa3e369a42693ef8/packages/@ember/-internals/glimmer/lib/helpers/readonly.ts#L118

The `readonly` template helper has been marked private since 2016. However, it does not seem like we deprecated it and it continues to work.

I see two options:

1. Restore the docs to public
2. Add a deprecation for intimate API until the next LTS

I do not believe using readonly is still necessary in any but very old code using ember component.

Read packages/@ember/-internals/glimmer/lib/helpers/readonly.ts around line 118 and inspect how the readonly template helper is currently marked. Resolve whether this issue should restore public documentation or introduce an intimate-API deprecation; done means the chosen policy is reflected consistently.

Written by the indexing model from the issue text.