Clarify static / const lifetime extension

What we want to say, for lifetime extension on `static` and `const`
items, is that the lifetime is extended to the end of the program when
the temporary would be extended, according to our other rules, to a
scope outside that of the initializer expression.  Let's take the easy
way out here and simply refer normatively to the other rule  for this.

Author: traviscross

src/destructors.md

@@ -336,9 +336,7 @@ println!("{}", x);
 ```
 
 r[destructors.scope.lifetime-extension.static]
-Lifetime extension also applies to the top-level scope of `static` and `const` items,
-where it makes temporaries live until the end of the program.
-For example:
+The temporary scopes for expressions in `const` and `static` item initializers are sometimes extended until the end of the program following the same rule as in [destructors.scope.lifetime-extension.let]. For example:
 
 ```rust
 const C: &Vec<i32> = &Vec::new();