[structure.specifications] remove "implementation-defined" from example

mentioning [[expects]] and add cross-reference to [[expects]] attribute.

Author: jwakely

source/lib-intro.tex

@@ -532,7 +532,7 @@
 that the function assumes to hold whenever it is called.
 \begin{example}
 An implementation might express such conditions
-via an implementation-defined attribute such as \tcode{[[expects]]}.
+via an attribute such as \tcode{[[expects]]}\iref{dcl.attr.contract}.
 However, some such conditions might not lend themselves
 to expression via code.
 \end{example}