This is a good suggestion, as it ensures that there is a single person who is responsible for maximizing the value of the product and managing the Product Backlog. Having one Product Owner also avoids confusion, duplication, or conflict among multiple Product Owners who may have different visions, priorities, or expectations for the product.
This is also a good suggestion, as it ensures that there is a single source of truth for the product requirements and that they are transparent and visible to all stakeholders. Having a single Product Backlog also enables the Product Owner to order the items based on their value, risk, dependency, and urgency, and to collaborate with the Scrum Teams to select and refine the items for each Sprint.

Explanation/Reference:

Explanation
User documentation is part of your Definition of Done, which means that it is a requirement for delivering a potentially releasable product increment. Your Scrum Team cannot ignore or postpone this requirement, even if there are not enough technical writers for all teams. Your Scrum Team is responsible for creating and maintaining the user documentation, and the Developers are the ones who have the skills and authority to do so. The Scrum Master can help the Developers learn how to write effective user documentation, and the Product Owner can provide feedback on its quality and value.
References:
Scrum Guide 2020, page 9: "The Definition of Done is a formal description of the state of the Increment when it meets the quality measures required for the product." Scrum Guide 2020, page 10: "The Developers are the people in the Scrum Team that are committed to creating any aspect of a usable Increment each Sprint." Professional Scrum Master II Course, page 12: "The Definition of Done is a crucial aspect of transparency. If it is not clear when work is done, then stakeholders may make wrong assumptions about the state of the software."

The Scrum Master is responsible for promoting and supporting Scrum as defined in the Scrum Guide. Scrum Masters do this by helping everyone understand Scrum theory, practices, rules, and values.
The Scrum Master is a servant-leader for the Scrum Team. The Scrum Master helps the Scrum Team understand which of their interactions are helpful and which aren't.
The Scrum Master helps everyone change these interactions to maximize the value created by the Scrum Team.

This is a good solution because it is consistent with the Scrum value of commitment, which means that the Scrum Team members are willing to learn new skills and take on new challenges to deliver value. By volunteering to take on this work, the Developers can reduce the dependency on Stella, increase their cross-functionality, and collaborate more effectively within and across the Scrum Teams.
This is another good solution because it is consistent with the Scrum principle of empiricism, which means that the Scrum Team learns and adapts based on the actual outcomes and feedback, rather than following a predefined plan. By asking Stella for a plan to hire and train additional people, the Scrum Master can help address the root cause of the dependency and create more capacity and diversity in Stella's domain. By working with the Product Owner and Scrum Teams to re-prioritize the work, the Scrum Master can also help optimize the value and feasibility of the product backlog items, and enable the Scrum Teams to deliver valuable and functional increments every Sprint.
This is also a good solution because it is consistent with the Scrum value of respect, which means that the Scrum Team members appreciate and value each other's skills and perspectives.
By creating a team with Stella, the Scrum Master can acknowledge and leverage Stella's expertise and experience in her domain. By identifying people from the Scrum Teams to temporarily work in Stella's domain, the Scrum Master can also facilitate knowledge transfer and skill development among the team members, which can improve their self-organization and collaboration.