If it was only an API reference, I might not hesitate to make it all public domain, since I don't believe in copyright on interfaces.
But the Nettle manual contains some more stuff than a plain API reference (even if it's the latter that is most relevant for Pike documentation). E.g., the discussion of randomness for cryptogaphic applications, http://www.lysator.liu.se/~nisse/nettle/nettle.html#Randomness, would make some sense for copying into a cryptography textbook or tutorial. Whether or not real problems with proprietary books or drm are *likely*, is a different matter.