“The purpose of this License is to make a manual, textbook,
or other written document “free” in the sense of freedom: to assure
everyone the effective freedom to copy and redistribute it, with or
without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a
way to get credit for their work, while not being considered
responsible for modifications made by others.”
“This License is a kind of “copyleft”, which means that
derivative works of the document must themselves be free in the
same sense. It complements the GNU General Public License, which is
a copyleft license designed for free software.”
“We have designed this License in order to use it for manuals
for free software, because free software needs free documentation:
a free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to software
manuals; it can be used for any textual work, regardless of subject
matter or whether it is published as a printed book. We recommend
this License principally for works whose purpose is instruction or
reference.”