It's not possible to add tags on documents published in sections but there's a UI bug, displaying the placeholder "Add tags to this document" even if no actions are available.
Steps to reproduce:
1. Create a Section
2. In a workspace, create a document and add tags to it
3. Publish the document in the section created at step 1
=> BUG
- The tags added at step 2 aren't displayed
- There's a UI bug displaying the placeholder mentioned above (see screenshot).