Improve translation to Dutch

Reproduction path:

1. Login using editor.
2. Open a document which is published. Take it offline and view one of the old version using `show revison history`
3. In another browser, login using author
4. Open the same document, edit (add or remove content) and request for publish
5. Go the editor browser, now click on restore. An exception is thrown.

The dutch message 'Kan actie niet uitvoeren op documenten met verzoeken.' is a bit cryptic for common users.

English gives 'Unable to perform action on document with pending requests' makes a bit more sense.

Suggestion: 'Kan deze actie niet uitvoeren op documenten met openstaande verzoeken.'

We should add a suggestion to the message (in both English as Dutch) to what the user could do to solve this situation.