Update `cmpnum` document.
`cmpnum` function may return `nil` on error.
@@ -1474,13 +1474,14 @@ cmpnum(mrb_state *mrb, mrb_value v1, mrb_value v2)
/* 15.2.9.3.6 */
/*
* call-seq:
- * self.f <=> other.f => -1, 0, +1
+ * self.f <=> other.f => -1, 0, +1, or nil
* < => -1
* = => 0
* > => +1
* Comparison---Returns -1, 0, or +1 depending on whether <i>fix</i> is
* less than, equal to, or greater than <i>numeric</i>. This is the
- * basis for the tests in <code>Comparable</code>.
+ * basis for the tests in <code>Comparable</code>. When the operands are
+ * not comparable, it returns nil instead of raising an exception.
*/
static mrb_value
integral_cmp(mrb_state *mrb, mrb_value self)
