Add comment about integers being positive

include/fplus/numeric.hpp

@@ -834,7 +834,7 @@ std::function<X(X)> divide_by(const X& x)
 }
 
 // API search type: div_pos_int_ceil : (a, a) -> a
-// Integer division, but rounding up instead of down.
+// Positive integer division, but rounding up instead of down.
 // div_pos_int_ceil(5, 3) == 2
 template<typename X>
 static auto div_pos_int_ceil(X numerator, X denominator) {