[swappable.requirements] Change "Requires" to "Preconditions" (#4886)

Author: brevzin

source/lib-intro.tex

@@ -1612,17 +1612,18 @@
 User code can ensure that the evaluation of \tcode{swap} calls
 is performed in an appropriate context under the various conditions as follows:
 \begin{codeblock}
+#include <cassert>
 #include <utility>
 
-// Requires: \tcode{std::forward<T>(t)} shall be swappable with \tcode{std::forward<U>(u)}.
+// Preconditions: \tcode{std::forward<T>(t)} is swappable with \tcode{std::forward<U>(u)}.
 template<class T, class U>
 void value_swap(T&& t, U&& u) {
   using std::swap;
   swap(std::forward<T>(t), std::forward<U>(u)); // OK: uses ``swappable with'' conditions
                                                 // for rvalues and lvalues
 }
 
-// Requires: lvalues of \tcode{T} shall be swappable.
+// Preconditions: lvalues of \tcode{T} are swappable.
 template<class T>
 void lv_swap(T& t1, T& t2) {
   using std::swap;