kinds: update documentation

src/libstd/kinds.rs

@@ -18,27 +18,21 @@
 They cannot be implemented by user code, but are instead implemented
 by the compiler automatically for the types to which they apply.
 
-The 2 kinds are
-
-* Send - owned types and types containing owned types.  These types
-  may be transferred across task boundaries.
-
-* Freeze - types that are deeply immutable.
-
 */
 
-#[allow(missing_doc)];
-
+/// Types able to be transferred across task boundaries.
 #[lang="send"]
 pub trait Send {
     // empty.
 }
 
+/// Types that are either immutable or have inherited mutability.
 #[lang="freeze"]
 pub trait Freeze {
     // empty.
 }
 
+/// Types with a constant size known at compile-time.
 #[lang="sized"]
 pub trait Sized {
     // Empty.