Add rust::String utf-16 constructors to website

Author: dtolnay

book/src/binding/string.md

@@ -23,6 +23,10 @@ public:
   String(const char *);
   String(const char *, size_t);
 
+  // Throws std::invalid_argument if not utf-16.
+  String(const char16_t *);
+  String(const char16_t *, size_t);
+
   String &operator=(const String &) noexcept;
   String &operator=(String &&) noexcept;