After consulting with FSF people (it's possible that Nettle will become a GNU package), I've more or less decided to make the entire manual public domain. It seems very difficult to combine a copyleft license such as the fdl (as long as there are no "invariant sections", I don't think the fdl causes any problems with debian) with permission to copy it into code and documentation under various other copyleft licenses as, e.g., in Pike (LGPL/MPL).
Tentative copyright notice:
This manual is placed in the public domain. You may freely copy it, in whole or in part, with or without modification. Attribution is appreciated, but not required.