Document timer thread.

asio/src/doc/overview/implementation.qbk

diff --git a/asio/src/doc/overview/implementation.qbk b/asio/src/doc/overview/implementation.qbk
index 63b3325..757ba6e 100644
--- a/asio/src/doc/overview/implementation.qbk
+++ b/asio/src/doc/overview/implementation.qbk
@@ -241,6 +241,10 @@
 `io_service::run()`, `io_service::run_one()`, `io_service::poll()` or
 `io_service::poll_one()`.
 
+* An additional thread per `io_service` is used to trigger timers. This thread
+is created on construction of the first `deadline_timer` or
+`deadline_timer_service` objects.
+
 * An additional thread per `io_service` is used for the `select`
 demultiplexing. This thread is created on the first call to `async_connect()`.