Project managers verbalize the big picture in a project plan. This high-level document covers the scope and requirements. Alone it doesn’t answer all questions so the plan is complemented by subsidiary plans. Project teams manage their work through a set of formal planning documents. They cover details about department functions. Scheduling, staffing, cost, quality, risk, acceptance and communications functions are created as subsidiary plans.
I recall only a few times in the last decade when a project required a documentation plan. Depending on whether technical documentation falls under marketing or R&D will determine who manages documentation planning.