[structure.specifications] Integrate [res.on.expects]. (#4042)

The latter consisted of a single sentence that is
best integrated into the place where the Preconditions:
element is introduced.

Also fixes LWG3168.

Author: jensmaurer

source/lib-intro.tex

@@ -620,7 +620,8 @@
 \item
 \expects
 the conditions
-that the function assumes to hold whenever it is called.
+that the function assumes to hold whenever it is called;
+violation of any preconditions results in undefined behavior.
 
 \item
 \effects
@@ -2976,12 +2977,6 @@
 This applies even to objects such as mutexes intended for thread synchronization.
 \end{note}
 
-\rSec3[res.on.expects]{Expects paragraph}
-
-\pnum
-Violation of any preconditions specified in a function's \expects element
-results in undefined behavior.
-
 \rSec3[res.on.requirements]{Semantic requirements}
 
 \pnum