Document rust::Slice's new constructor in book

Author: dtolnay

book/src/binding/slice.md

@@ -23,6 +23,9 @@ public:
   Slice(const Slice<T> &) noexcept;
   Slice(T *, size_t count) noexcept;
 
+  template <typename C>
+  explicit Slice(C& c) : Slice(c.data(), c.size());
+
   Slice &operator=(Slice<T> &&) noexcept;
   Slice &operator=(const Slice<T> &) noexcept
     requires std::is_const_v<T>;