As a first suggestion, drop the idea of using the FDL. Firstly, and most importantly, you'd need the permission of everybody who contributed copyrightable material towards the document. You don't have that.
Technically, of course, you'll need that permission no matter WHAT license you release it under. That just brings us onto the second problem, which is why getting that permission is going to be near impossible...
...beacuse the FDL is far, far too restrictive, and doesn't accomplish half what people think it does.
Just to start with, if I download Gears into my My Documents folder (home folder, if I were running Linux), I've just violated the FDL. Copy it onto a flash drive to take around to a friends house? Depending on the type of flash drive, also an FDL violation. Print part of it out for a friend? FDL violation.
Think about what you're trying to achieve. And also that you'll have to convince everybody who provides content for the document to agree with you. If you just want a license that makes it obvious people can distribute it freely, use the
BSD license. If you want to force people to keep future versions of it open, consider a
Creative Commons license, or if you're feeling evil,
the GPL. Using the FDL is just like declaring you're
trying to catch people out and make them break the license.
