They exist, and are very valuable, but I've never worked at a company that employed even one full time technical writer. If I go on a caffeine bender and write the 20 page doc OP described, nobody is coming back around to clean that up for me. At best some peers might tell me "hey, great effort there, but could you cut the word count by 75%?" Access to full-time professional editors seems like a luxury of very large organizations.