Not all of Twisted's documentation is bad. And if folks can't figure out the docs, Jean-Paul and Glyph are available to help in IRC and know everything about Twisted (since as far as I know they represent 90%+ of the current contribution and maintenance).
Besides, Tornado can afford to have (seemingly) more comprehensive documentation because it is far smaller in scope and support. Twisted has a large number of robust libraries for doing all sorts of things asynchronously, from web servers to ORMs to SSH and everything in between. It's like comparing an orange to a bushel of apples and it's silly. Twisted has never supplied a web framework, which is exactly what Tornado supplies.
The Twisted docs are lacking in many ways, I just don't understand how Tornado is objectively better because it doesn't currently have that problem -- they aren't the same thing!