Writing user manuals is
hard in my experience.
My suggestion -- something that I feel has helped me a lot -- is to use a machine-checkable format, then write scripts to check everything you can. For example, write a script that checks that every menu item is documented, and every dialog box, etc. Make sure the tests run both ways -- i.e., confirm that every documented item is actually in the software. Then run these scripts as part of your application
test suites. This way, when the software changes, you'll get an automatic report of where the documentation needs to be fixed.
I've used various XML-based formats for this in recent years (DocBook, homebrew), and I've used TeX in the past. Anything easy to parse is good. I guess even
Word is OK if you can write scripts in Visual Basic or something that can examine Word files -- I'm not sure if this is possible or not.