Add a doc comment explaining what convert_down() does

types/src/number/mod.rs

@@ -22,6 +22,7 @@ type Int = i64;
 type Flt = f64;
 
 trait Number: Debug + IsBool + IsChar + IsNumber + Value {
+    /// Convert a Number to the next lowest type in Scheme's number pyramid, if possible.
     fn convert_down(&self) -> Option<Box<Number>>;
 }