The Free Software Foundation is recommending what licenses to use for your software and documentation projects. They have recommended the GNU Free Documentation License as a dandy thing that is just marvellous for your project. They are dangerously and stupidly wrong.
tl;dr: Reusing GFDL content is unworkably unclear. When you ask the FSF about the tricky bits, they say “better get a lawyer, son.” This egregiously misses the entire damn point. Freedom only for those with copyright lawyers on tap is not freedom.
The GFDL is possibly one of the worst free content licenses ever. The only reason it has not been justifiably buried at the bottom of a swamp with “MISERABLE FAILURE” burnt into its forehead with a soldering iron is because Wikipedia used to use it. Wikipedia used it because Nupedia used it. Nupedia used it only because the CC licenses hadn’t been invented yet.
Literally no one understands how to reuse GFDL content safely, including the FSF. I sent a query about how it applies to aggregates; three months later, the FSF cut’n'pasted their “we have no idea either” response, suggesting you read the license text and consult your attorney. Given Wikimedia’s attorney at the time was Mike Godwin and it made his head hurt too, this strongly suggests no-one left at the FSF wants to think about this thing. (They finally clarified this particular issue in GFDL 1.3.)
In the context of mirroring a widely-edited wiki or a page thereof, its terms are difficult to follow, legally unclear and may be technically impossible to comply with in a comparable degree of safety to the GPL or CC by-sa. (Every copy must have the full 23 kilobytes of licence text attached, about seven pages of single-spaced 12 point. This is not so good for single articles or photographs, and Internet video is likely impossible to reproduce under the GFDL in legal safety. This is, of course, the easiest term of the GFDL to obey. CC by-sa allows the license to be named by reference.) And don’t even ask about images.
Before Wikimedia went Creative Commons, tedious nerds would frequently claim that any given reuser of Wikipedia content was technically violating the GFDL no matter what shrubberies they obtained (thus putting off quite a lot of reusers). And no-one, particularly not the FSF, could provide meaningful advice that could actually be relied upon, because the licence is deeply and fundamentally not fit for purpose.
The GFDL is so unremittingly awful, hard to trust legally and onerous in practice that the Debian project went so far as to throw its hands in the air and declare that it failed the Debian Free Software Guidelines. (The DFSG threat model is “What if an insane or malicious copyright holder comes after a reuser?” This is the question the FSF answers “Better get a lawyer, son.” THIS IS NOT HOW TO BE REUSABLE. The FSF does not answer this concerning the GPL, for instance.) So if you want your open source package in Debian, you probably don’t want the documentation under GFDL.
Use CC by-sa, CC-by or Public Domain, like approximately everyone else in the whole goddamn free content world does. If it’s a software manual, licence or dual-licence it under the same licence as the software itself.
You know how some people use shitty handrolled software licenses that are technically free or open but aren’t actually compatible with anything else, and everyone else thinks they’re dicks for it and avoids using their stuff? That’s why you shouldn’t use the GFDL either.