Shorten, yet clarify, initial summary sentences

Author: carols10cents

src/libcore/clone.rs

@@ -46,7 +46,9 @@
 
 use marker::Sized;
 
-/// A common trait for cloning an object. Differs from `Copy` in that you can
+/// A common trait for the ability to explicitly duplicate an object.
+///
+/// Differs from `Copy` in that you can
 /// define `Clone` to run arbitrary code, while you are not allowed to override
 /// the implementation of `Copy` that only does a `memcpy`.
 ///

src/libcore/default.rs

@@ -84,7 +84,9 @@
 
 use marker::Sized;
 
-/// A trait for giving a type a useful default value. For more information, see
+/// A trait for giving a type a useful default value.
+///
+/// For more information, see
 /// [the module-level documentation][module].
 ///
 /// [module]: ../../std/default/index.html