User documentation is part of your Definition of Done.

The Scrum Guide states that “the Developers are the people in the Scrum Team that are committed to creating any aspect of a usable Increment each Sprint”. It also states that “the Developers can do the work of any other role on the Scrum Team, as long as accountability is met”. Therefore, the Developers on the Scrum Team should write the user documentation, as it is part of the Definition of Done and the work needed to deliver a “Done” Increment. The other options are not acceptable, as they violate the Scrum values of commitment, focus, and respect, and they create dependencies, delays, and waste.

References:

The Scrum Guide, section 2.2. Developers

Professional Scrum Product Owner I (PSPO I) Learning Path, section 2.3. The Product Owner and the Definition of Done

Professional Scrum Product Owner I (PSPO I) Sample Questions, question 12