Add missing documentation for use_future_t::allocator_type.

asio/include/asio/use_future.hpp

diff --git a/asio/include/asio/use_future.hpp b/asio/include/asio/use_future.hpp
index f1722e1..f883eef 100644
--- a/asio/include/asio/use_future.hpp
+++ b/asio/include/asio/use_future.hpp
@@ -41,6 +41,8 @@
 class use_future_t
 {
 public:
+  /// The allocator type. The allocator is used when constructing the
+  /// @c std::promise object for a given asynchronous operation.
   typedef Allocator allocator_type;
 
   /// Construct using default-constructed allocator.