Documentation in whatever form is part of user experience design.
My approach to getting things done is underpinned always with effective relationships across disciplines. I use whatever approach is in place, or I use my own framework which includes broadly:
• Identify stakeholders, including users
• Review requirements and existing documentation
• Validate or create content strategy and tools
• Create high-level roadmap or plan
• Capture writing and design brief
• Set up meetings and project infrastructure
• Research and learn
• Create templates and content architecture
• Write & design; review and repeat