The MuseScore community maintains a collaborative written handbook. Any logged in user can edit and improve the pages of the handbook. If you see a mistake or something missing in the handbook, don't hesitate to fix it or extend it—click the ⠇ to the right of the title, choose "Edit", and improve the page.
You can also add to the community's collection of "How to" pages. If you record screen capture videos and upload them to YouTube, you can insert a video into a How To.
The handbook is written in English and translated into other languages. If you would like to help with the translation see the instructions here.
In cases where you would like to discuss changes, post in the documentation forum.