Revert "Omit swap member functions from website"

This reverts commit d835ac6.

Author: dtolnay

book/src/binding/box.md

@@ -34,6 +34,8 @@ public:
   template <typename... Fields>
   static Box in_place(Fields &&...);
 
+  void swap(Box &) noexcept;
+
   // Important: requires that `raw` came from an into_raw call. Do not
   // pass a pointer from `new` or any other source.
   static Box from_raw(T *) noexcept;

book/src/binding/string.md

@@ -51,6 +51,8 @@ public:
   bool operator<=(const String &) const noexcept;
   bool operator>(const String &) const noexcept;
   bool operator>=(const String &) const noexcept;
+
+  void swap(String &) noexcept;
 };
 
 std::ostream &operator<<(std::ostream &, const String &);

book/src/binding/vec.md

@@ -57,6 +57,8 @@ public:
   const_iterator end() const noexcept;
   const_iterator cbegin() const noexcept;
   const_iterator cend() const noexcept;
+
+  void swap(Vec &) noexcept;
 };
 #
 # template <typename T>