Document the Zero trait

Author: erickt

doc/rust.md

@@ -1717,7 +1717,8 @@ Supported traits for `deriving` are:
 * `Clone` and `DeepClone`, to perform (deep) copies.
 * `IterBytes`, to iterate over the bytes in a data type.
 * `Rand`, to create a random instance of a data type.
-* `Zero`, to create an zero (or empty) instance of a data type.
+* `Default`, to create an empty instance of a data type.
+* `Zero`, to create an zero instance of a numeric data type.
 * `ToStr`, to convert to a string. For a type with this instance,
   `obj.to_str()` has similar output as `fmt!("%?", obj)`, but it differs in that
   each constituent field of the type must also implement `ToStr` and will have

doc/tutorial.md

@@ -2249,7 +2249,7 @@ enum ABC { A, B, C }
 
 The full list of derivable traits is `Eq`, `TotalEq`, `Ord`,
 `TotalOrd`, `Encodable` `Decodable`, `Clone`, `DeepClone`,
-`IterBytes`, `Rand`, `Zero`, and `ToStr`.
+`IterBytes`, `Rand`, `Default`, `Zero`, and `ToStr`.
 
 # Crates and the module system