GFDL invariant sections are non-free

is licensed under the GNU Free Documentation License with invariant sections, which are non-free because they cannot be modified

When you say non-free I assume you are using the FSF definition of free software, but documentation is not software. We follow the FSF policy regarding free documentation http://www.gnu.org/philosophy/free-doc.html which say:

While a blanket prohibition on modification is unacceptable, some kinds of limits on the method of modification pose no problem. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are OK. It is also no problem to require modified versions to include notice that they were modified, even to have entire sections that may not be deleted or changed, as long as these sections deal with nontechnical topics. (Some GNU manuals have them.)

However, it must be possible to modify all the technical content of the manual, and then distribute the result through all the usual media, through all the usual channels; otherwise, the restrictions do block the community, the manual is not free, and so we need another manual.

So, a manual can contain invariant sections and still be free documentation, but it can also apply the invariant sections wrong and be non-free documentation. Since all the packages related to this issue in Trisquel are GNU packages, we know they were already reviewed by the FSF to meet the freedom criteria above.

In any case, if you find any non-free documentation in Trisquel (either under a non-commercial or similarly restrictive license, or with a misuse of the invariant section clauses) please report it and we will remove the files and warn the free distros community about it.