Rather than being defined as !positive and !negative, these should act the
same as negative and positive (respectively). The only effect of this change
should be that all four functions will now return false for NaN.
Naturaldocs isn't really that great but it seems easier to get
something working than with doxygen, for which we would need to
convert rust code to something C++ish. We probably want to just
write a rustdoc utility at some point.
