Alert that no fields can be re-used?

Problem/Motivation

If a user wants to try the new design from 🐛 Improve re-use an existing field user experience Fixed , and opens the Article "Manage fields" page, the button is not shown because all fields are used in this content type, so there are none to re-use.

This makes sense, but can cause the user to think that the new feature is not working.

Perhaps an alert like "All fields are used in this content type, none to re-use." could be displayed, in that situation?

Steps to reproduce

Want to try the new feature from 🐛 Improve re-use an existing field user experience Fixed by opening the Article Manage fields page (/admin/structure/types/manage/article/fields) expecting a button, like this:

Result
Don't see a "Re-use an existing field" button ...

Solution?
Inform that no fields can be re-used?

Proposed resolution

Maybe show an alert text, like "All fields are used in this content type, none to re-use." just to inform about the situation, and why there's no "Re-use an existing field" button?

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet