Saying "you can modify the documentation, just not the invariant
sections" isn't enough, incidentally, to answer this, because the
attachment of the invariant section to the documentation *is* a
property of the documentation. (Consider if there were such a thing
on a piece of software: you can modify all these functions, but not
that one. The result is that the whole thing is nonfree.)
This would be like having a technical section which can't be changed.
That would be non-free, but it's not what our actual invariant sections do.
For them, the preamble of the GPL is a better analogy.
That too is text that can't be changed or removed from the program,
but does not restrict its practical functionality.
