Add a note to clarify that concurrent invocation is not guaranteed for
handlers in different strands.

asio/include/asio/strand.hpp

diff --git a/asio/include/asio/strand.hpp b/asio/include/asio/strand.hpp
index 48543d6..96af565 100644
--- a/asio/include/asio/strand.hpp
+++ b/asio/include/asio/strand.hpp
@@ -72,6 +72,9 @@
  * happens-before the other. Therefore none of the above conditions are met and
  * no ordering guarantee is made.
  *
+ * @note The implementation makes no guarantee that handlers posted or
+ * dispatched through different @c strand objects will be invoked concurrently.
+ *
  * @par Thread Safety
  * @e Distinct @e objects: Safe.@n
  * @e Shared @e objects: Safe.