doc: Clean up primitive short descriptions

This makes the primitive descriptions on the front page read properly
as descriptions of types and not of the associated modules.

src/libcore/ptr.rs

@@ -10,7 +10,7 @@
 
 // FIXME: talk about offset, copy_memory, copy_nonoverlapping_memory
 
-//! Operations on raw pointers, `*const T`, and `*mut T`.
+//! Raw, unsafe pointers, `*const T`, and `*mut T`
 //!
 //! Working with raw pointers in Rust is uncommon,
 //! typically limited to a few patterns.

src/libcore/tuple.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Operations on tuples
+//! A finite heterogeneous sequence, `(T, U, ..)`
 //!
 //! To access a single element of a tuple one can use the `.0`
 //! field access syntax.