add comment explaining arbitrary gen for uint

src/uint.rs

@@ -1372,11 +1372,16 @@ macro_rules! impl_quickcheck_arbitrary_for_uint {
 				let mut res = [0u8; $n_bytes];
 
 				let p = g.next_f64();
+				// make it more likely to generate smaller numbers that
+				// don't use up the full $n_bytes
 				let range =
+					// 10% chance to generate number that uses up to $n_bytes
 					if p < 0.1 {
 						$n_bytes
+					// 20% chance to generate number that uses up to $n_bytes / 2
 					} else if p < 0.2 {
 						$n_bytes / 2
+					// 70% chance to generate number that uses up to $n_bytes / 5
 					} else {
 						$n_bytes / 5
 					};