Publicize: add explanation when toggles are disabled for a published post

#### Steps to reproduce the issue

1. Start from a site where Publicize is enabled and some Social Media accounts are connected.
2. Publish a new post; Publicize will be triggered and tweets / Facebook messages will be posted.
3. Open the Published post in the block editor or classic editor again.
4. The Publicize options are greyed out.

The options are greyed out because the post has already been published, and cannot be publicized again. However, that is not a limitation authors may know about. It would be nice to offer more details in the post editor so folks understand why they cannot publicize their post again.

Related: #17281

Start by reproducing the behavior in the block editor and classic editor described in the issue, then trace the Publicize controls for an already published post. Add an explanation explaining why the toggles are disabled, and verify that it appears in both editors while the controls remain unavailable.

Written by the indexing model from the issue text.