Rui Miguel Silva Seabra summed it up very well.
Why is different the "free" as in freedom concept for documentation from the concept of "free" as in freedom for "software"?
It isn't that different, the four freedoms still apply. The difference is that the content isn't a functional work, and one may wish to attach a dedication to the text, or maybe something else that isn't related at all to the actual text.
But there are problems with this. If every contributer/derivator adds a dedication, then we can end with lots of dedications for a relatively small piece of work.
And if you lots of people modify a small program, and do not collect copyright assignments, then you will have a long list of copyright holders. Nothing different from dedications.
You can pretend to like fluffy puppies, but don't pretend that you like them more than more. Because then it is simply not true.
You are arguing that I can rewrite whatever I write, to whatever I wish to. Since otherwise, it is simply not `free'. Well, here I did exactly that. Here are the exact modifications I made:
You [-can-] {+cannot+} pretend to [-like fluffy puppies, but don't-] {+do a free document, and then+} pretend that [-you like them more than more.-] {+it has a part that can't be striped and modified.+} Because then it is [-simply-] not [-true.-] {+free.+}
If we allow for the modification of what is written, at all costs, then we cannot have a serious discussion, ever. We cannot express what we think without the fear that someone will claim that one claims something else, etc. I took the liberty to modify what you wrote, to explain to you how utterly silly, and devastating it is to allow such things. Nobody should ever have this right.
I also agree that it makes sense that some philosophy text, music recordings or images can't be changed. But IF they can't be changed, they are NOT free. Let's not pretend that everything must be free and next change the meaning of freedom.
They aren't free _software_, but they are free texts/recordings/images. Nobody is redefining the meaning of freedom here. You are on the other hand trying to redefine freedom to mean `free software'.
You aren't limited anywhere when you modify free documentation of a free program. This is like saying that you are limited by the GPL to create non-free works, which is simply nonsense.
Sorry, but if the documentation of a free program has FDL, then it can contain invariant sections, so that I am limited :)
You aren't limited, you are free to change the main work of the document; nobody stops you from doing this.
Cheers.