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/libstd/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 the _N_-th element of a tuple one can use `N` itself
 //! as a field of the tuple.