Is documentation for a GUI app different from documentation for a web site? That's a curly question someone asked me recently.
I only considered the documentation in the completed app or web page, not documentation for the development process. Alas, my thinking did not produce any great new truths.
I believe there are no profound differences. The problem is that most people don't understand the requirements for good web content, so that statement isn't very enlightening! Here are some tips from my perspective. They're general points, but I'd give them priority.
And enjoy your work!
The Thinker, sculpture by Auguste Rodin. Picture taken in Musee Rodin in Paris, France by Pufacz.