I've done something like this once, neatly grouped in chapters, with screenshots and even short video-captures. Everything the client needed was covered. Problem was... that particular client mostly forgot to look it up in the first place - even though that manual wasn't hidden away in some obscure place, or distributed as a separate file. It lived right there in the PW admin as a custom admin page.
I'd share it here, but a) it was in german, b) the site's setup is way too custom-tailored to make sense for most other people and c) it uses pro modules like Form Builder and Lister Pro. Also, afair the screenshots contain sensitive information, and the rich-text editor in use was CKE with a non-standard choice of buttons.
I'm not sure I would ever do such a manual again, seeing how much work went into it, and how easily it can be overlooked. I would invest my time even more into UX; e.g. for some sites, it makes sense to completely hide the page-tree. Also, use field labels and hint texts liberally. Hide, rename or otherwise simplify admin stuff (hooks? Admin On Steroids - or whatever the successor is called now? there's plenty of tools and examples around). Some stuff is right there in the core, e.g. restrict templates by page-tree, or disable changing the template etc.