Lionel Elie Mamane lionel@mamane.lu
It isn't. What they require is that the documentation be released under the GFDL or a compatible license. You can dual (or triple) license it under GFDL|GPL or GFDL|BSD or ...
However, you'll not be allowed to copyleft it under GPL alone. Savannah seeks to require all projects to permit others to add odious uneditable sections to their manuals and forbid users from removing them again.
That's apart from the other problems with the current FDL.