Sure, happy to elaborate on that.
The GNU project repeatedly talks about documentation and other works as somehow separate from software, and makes comments like this: "However, for manuals and textbooks, or more generally any sort of work that is meant to teach a subject, we recommend using the GFDL rather than the GPL.". See this FAQ entry for a more extensive example: https://www.gnu.org/licenses/gpl-faq.html#WhyNotGPLForManual... . (Disclaimer again: I disagree entirely with just about everything said in that entry, as do numerous other FOSS developers.)
In general, the GNU project doesn't care much about things other than software. In particular, they've said they don't care much about the ability to modify data (such as media for a game), and they actively oppose modifiability of non-technical writing (such as their own essays, which allow unmodified copies only).
Among other things, the GFDL allows manuals to contain extensive unremovable and unmodifiable sections, such as the GNU Manifesto (which many people mostly agree with and find inspiring) and "Free Software Needs Free Documentation" (which many people strongly disagree with due to it propagating many of the same advocacy of the GFDL). The GFDL also contains very particular requirements about text placed on the front and back covers of a printed book (beyond just attribution), which also does not permit modification. (That requirement seems particularly hypocritical given the FSF's own essay about the GPL-incompatible 4-clause BSD license with advertising clause, https://www.gnu.org/philosophy/bsd.html .)
Most importantly, the GFDL and GPL remain incompatible licenses, which means you can't copy anything between documentation and code.
All of the above contributes to my recommendation that you ignore anything the GNU project says about the GFDL, and anything they say about not using the GPL on manuals.
All that said, I do actually consider almost all of the GNU project's "philosophy" section (https://www.gnu.org/philosophy/) well worth reading; it certainly inspired me when I first started learning about Free Software. Just take some of it with a grain of salt, particularly when it comes to anything other than software, because the GNU project really doesn't care about anything other than software.
Fortunately, the most recent update to the GFDL provided a way for Wikipedia and other large wikis to transition away from the GFDL and to the Creative Commons Attribution Share-Alike (by-sa) license, which doesn't have the issues with Invariant Sections and Cover Texts. Creative Commons licenses remain GPL-incompatible, which can cause practical problems for anything that might mix with other FOSS work, but not for Wikipedia and other similar wikis. Many people avoid them for software documentation for that reason, but they seem like perfectly reasonable licenses for almost any other work.
Recommendations for non-software: For documentation or other bits associated with a software project, use the same license as that software project. (For instance, X11 documentation uses the all-permissive MIT license, and the documentation for most non-GNU GPLed projects uses the GPL.) For any other work, use the GPL if some clearly identifiable "source" form exists that you want to preserve access to, use CC-by-sa if not but you still want a copyleft, and use CC-by or a simple all-permissive license like MIT otherwise.